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So what do people think?

Personally, I am quite neutral on the subject.

It was extremely necessary, given the situation the Imperium was in.

Coolyo294 wrote:It was extremely necessary, given the situation the Imperium was in.

/thread

Brother Coa wrote:

Coolyo294 wrote:It was extremely necessary, given the situation the Imperium was in.

/thread

I would just like to add that the Codex Astarte should not be followed to the letter but should be used as a guide/sugestion.

Magnus Calgar : "Reliable is also predictable"

They didn't have to de-fang the Chapters as much as they did. I think making a Codex chapter 5,000 or 10,000 would have sufficed fine since pre-Heresy legions were often in the 100,000 range (such as Word Bearers). It really makes no sense that a few hundred space marines could do the kind of missions required of them across entire planets. Yes, I know they're shock airborne troops and rely on the Guard for protracted missions. Yes, I know they're uber elite. But given the limitations they've shown with their missions often taking place across an entire planet or even star system, it is still far too vast for a few squads of a 100 astartes.

GW has problems with scale. They made this vast galaxy and yet the High Lords of Terra approve Officio Assassinorum assassination, major SoB orders are several thousand strong, a typical IG regiment is often under 5,000 men, etc,

What is this question asking? Considering we know almost nothing about what is actually contained in the Codex, how can it be hated unless we are overzealous and strangely delusional Black Templars or Space Wolves players who actually think we are Space Marines.

I share the views of Dorn, Russ and Vulkan. Funnily enough they are also my 3 favourite primarchs/legions.

Toilet paper written by a coward.

YMMV

Agree with the Codex. It was necessary since without the Emperor to reign over them, another rebellion would probably have erupted as the Primarchs would have fought over the ruined Imperium within decades of His incapacitation. And Guilliman himself said that the Codex was not holy writ to be followed to the letter, but rather guidelines on organization and tactics. That's why each Chapter has it's own unique traditions and organizational differences.

I think it was totally necessary, the Imperium would have gone balls up without it. But it's not like I'm biased towards Ultramarines or anything, I mean they just happen to be the epitome of a Space Marine chapter, the legion that Horus had to send to the other end of the Galaxy so he could start the Heresy, have a very suave looking colour scheme and the coolest of all Primarchs, I don't see YOU publishing a novel Rogal Dorn!

Sir Samuel Buca wrote:I think it was totally necessary, the Imperium would have gone balls up without it. But it's not like I'm biased towards Ultramarines or anything, I mean they just happen to be the epitome of a Space Marine chapter, the legion that Horus had to send to the other end of the Galaxy so he could start the Heresy, have a very suave looking colour scheme and the coolest of all Primarchs, I don't see YOU publishing a novel Rogal Dorn!

Oh, sorry, Rogal Dorn didn't get the book memo, he was too busy defending Terra with his Legion...


As a devout Black Templars player, I can say that I honestly don't care if other Chapters use it: I don't use it anyway!

Halmyr wrote:

Brother Coa wrote:

Coolyo294 wrote:It was extremely necessary, given the situation the Imperium was in.

/thread

I would just like to add that the Codex Astarte should not be followed to the letter but should be used as a guide/sugestion.

Magnus Calgar : "Reliable is also predictable"

Thank you Graham McNeill. Of course the Codex wasn’t a strict step-by-step rulebook before he got his paws on it and turned the Ultramarines into idiots.

The way McNeill writes Ultramarines it’s hard to see how they could have survived past their first century by following this flawed doctrine strictly, a doctrine that seemingly proves detrimental every other day.

Thatguy91 wrote:I share the views of Dorn, Russ and Vulkan. Funnily enough they are also my 3 favourite primarchs/legions.

Actually Vulkan is retconned to be a Guilliman supporter in Imperial Armor 10.

AlmightyWalrus wrote:Oh, sorry, Rogal Dorn didn't get the book memo, he was too busy defending Terra with his Legion...

He was only defending Terra because he couldn't be trusted to do anything important like conquer the galaxy and nobody expected Terra to be attacked anyway.

All jokes aside, the Codex was written after the Battle of Terra anyway. If Dorn didn't have much influence on it, that was because he took off crusading. Guilliman, on the other hand, managed to put the galaxy back together more efficiently, repair Terra, reorganize the Imperium's government in the absence of it's thousands of years old autocratic Emperor, and still write an exhaustive galactic best selling novel.


Automatically Appended Next Post:

Gree wrote:Thank you Graham McNeill. Of course the Codex wasn’t a strict step-by-step rulebook before he got his paws on it and turned the Ultramarines into idiots.

The way McNeill writes Ultramarines it’s hard to see how they could have survived past their first century by following this flawed doctrine strictly, a doctrine that seemingly proves detrimental every other day.

McNeill is one of the worst writers on the GW staff. I think the only novel of his that I liked was A Thousand Sons, and that was only because the human Remembrancer characters were somewhat interesting. Right up until he forgot about them at the end, lol.

I think at the time of writing it was a useful tool for the Imperium and its Space marines. After the Heresy some form of order was needed to stabilise the legions. In the case of the Iron Hands and Blood Angels I'd argue it was vital to prevent a legion being under the command of an 'ordinary' Space Marine, given the death of their respective Primarchs. Plus having the distilled wisdom of one of the greater tactical minds of the Imperium can never be a bad thing.

However in the current climate and state of things in the Universe, I think it can be a critically damaging thing. While I am in no doubt it still contains useful information, I don't see that it can cope with the myriad of situations that will crop up in a galaxy filled with foes that could not be forseen in the 31st Millenium. Necrons, Tyranids and Tau spring to mind. The perceived rigidity of codex useage (irrespective of its intended use and application) whereby if you don't follow it 'you can't be a true Space Marine' or somesuch, means a great deal of 'new' tactical avenues may be missed or passed over in favour of 'outdated' and less optimal ones.

As others have said, without actually seeing the Codex we can't know for sure exactly how good or bad it is. But based off what we do know, it is by no means perfect. Room for improvements exist, such as bulking up Chapter numbers for more effective crusades and defense of the Imperium-more akin to the legions of old without the risk of one or two bringing the Imperium to its knees.

Anyway, thats just my view

Veteran Sergeant wrote:

AlmightyWalrus wrote:Oh, sorry, Rogal Dorn didn't get the book memo, he was too busy defending Terra with his Legion...

He was only defending Terra because he couldn't be trusted to do anything important like conquer the galaxy and nobody expected Terra to be attacked anyway.

All jokes aside, the Codex was written after the Battle of Terra anyway. If Dorn didn't have much influence on it, that was because he took off crusading. Guilliman, on the other hand, managed to put the galaxy back together more efficiently, repair Terra, reorganize the Imperium's government in the absence of it's thousands of years old autocratic Emperor, and still write an exhaustive galactic best selling novel.

The Codex was actually written before the Siege of Terra in the early stages of the Heresy, rather than Guilliman choosing to aid the Emperor he chose to hang back and write the Codex Astartes instead.

Again, why I dislike McNeill.

Storm Lord wrote:I think at the time of writing it was a useful tool for the Imperium and its Space marines. After the Heresy some form of order was needed to stabilise the legions. In the case of the Iron Hands and Blood Angels I'd argue it was vital to prevent a legion being under the command of an 'ordinary' Space Marine, given the death of their respective Primarchs. Plus having the distilled wisdom of one of the greater tactical minds of the Imperium can never be a bad thing.

However in the current climate and state of things in the Universe, I think it can be a critically damaging thing. While I am in no doubt it still contains useful information, I don't see that it can cope with the myriad of situations that will crop up in a galaxy filled with foes that could not be forseen in the 31st Millenium. Necrons, Tyranids and Tau spring to mind. The perceived rigidity of codex useage (irrespective of its intended use and application) whereby if you don't follow it 'you can't be a true Space Marine' or somesuch, means a great deal of 'new' tactical avenues may be missed or passed over in favour of 'outdated' and less optimal ones.

Except it is updated. From the 5th Edition Codex:

The Codex Astartes further defines the tactical roles, equipment specifications and uniform identification markings of the Space Marines. These guidelines have evolved over the centuries, and the Codex Astartes of the forty first millennium is a highly developed treatise combining the wisdom of hundreds of military thinkers throughout history.

In order new stuff to the Codex it first must exist outside the Codex to be added.

In addition the Deathwatch First Founding supplement explains that Guilliman started the first volumes of the book and Cahapter Masters and Captains across history have added their thoughts to the book.

Unfortunately many players and some authors, Grahman McNeill included, seem to be under the impression the Codex is never updated and not very useful in modern times. Reading McNeill’s books I get the impression that anything not explicitly described in the Codex is a problem for the Ultramarines, yet we know new things are added. I blame the Ultramarines series of books for giving a negative view of the Codex Astartes, a book that will at some times explicitly give you bad advice at points and sometimes you simply have to go against.

I think the splitting of the chapters is no longer relevant. In the 41st millennium it simply isn't possible for any space marine legion to single-handily amass so much power. With the IG and the Navy split up they can't collectively be corrupted, and also no one "worships" the Space Marines like people did before the Horus Heresy. They are feared due to their mysteriousness, and respected due to their prowess, but very few entire armies are going to bow down to a space marine and forsake the Emperor.

So even if every chapter in the IoM rebelled at the same time against the Imperium, they would have to deal with: The IG, the Navy, the Eclissiarchy and its army of SoB, the entirety of the Inquisition, and the Mechanicum. The IG and Navy alone would annihilate every chapter by themselves if it came down to it (though obviously at a great cost).

So the chances of mass SM rebellion on the scale of the Horus Heresy is practically zero. That being the case, I say let the chapters grow to legion size again.

A thousand Chapters of 100,000+ Space Marines is a lot of Space Marines, lol.

Althought we have no idea was is written in the codex, from what I understand, outlines a aggresive form of combat, lighting warfare.
To use the minimum amount of ressources for the maximum level of effectivess.
Basicly a futuristic form of "art of war" by sun Tzu.

Most chapter look at it with advice but can think beyond it and adapt to situation.
The Codex becomes a problem when it is taken to the word, like the way Graham Mcneil portrais the Ultramarine.
Yes its a reliable way to fight you enemy, but as soon as the enemy knows how you operate, your pretty much done.

It seams to have some good pointers, but a little too restrictive. Chapters that don't follow it to the letter (BA,SW,DA) seam to be doing just fine.

Halmyr wrote:Althought we have no idea was is written in the codex, from what I understand, outlines a aggresive form of combat, lighting warfare.
To use the minimum amount of ressources for the maximum level of effectivess.
Basicly a futuristic form of "art of war" by sun Tzu.

Most chapter look at it with advice but can think beyond it and adapt to situation.
The Codex becomes a problem when it is taken to the word, like the way Graham Mcneil portrais the Ultramarine.
Yes its a reliable way to fight you enemy, but as soon as the enemy knows how you operate, your pretty much done.

The problem is, McNeill's representation of it seems to conflict with everything else we know about it, and he even contradicts himself at times.

Somebody "following the codex" isn't going to be predictable. That's the exact opposite of what the Codex would teach its students to do. It isn't a list of instructions for combat in an "If A, then B" format. The strategy and tactics portion treatise on every conceivable combat scenario, and hundreds of pages of examples and options. Gives the commander on the ground ideas to use in every kind of situation, so they can rely on the experience of thousands of years of warfare in addition to their own initiative. Think about it. Modern military commanders don't have anywhere near that kind of knowledge to draw on, and we've still had military geniuses in the last hundred years. A Space Marine commander well versed in the wisdom of the Codex is like all of them wrapped into one, and more. Sure, the Codex also has boring, regimented stuff like how a Chapter is supposed to be organized, and the way training is supposed to be conducted, and how to manage gene seed reserves, etc. But that's not the bulk of it.

Vet Sarge has a point.

The problem is, McNeill's representation of it seems to conflict with everything else we know about it, and he even contradicts himself at times.

Somebody "following the codex" isn't going to be predictable. That's the exact opposite of what the Codex would teach its students to do. It isn't a list of instructions for combat in an "If A, then B" format. The strategy and tactics portion treatise on every conceivable combat scenario, and hundreds of pages of examples and options. Gives the commander on the ground ideas to use in every kind of situation, so they can rely on the experience of thousands of years of warfare in addition to their own initiative. Think about it. Modern military commanders don't have anywhere near that kind of knowledge to draw on, and we've still had military geniuses in the last hundred years. A Space Marine commander well versed in the wisdom of the Codex is like all of them wrapped into one, and more. Sure, the Codex also has boring, regimented stuff like how a Chapter is supposed to be organized, and the way training is supposed to be conducted, and how to manage gene seed reserves, etc. But that's not the bulk of it.

Well, without knowing the full contents of the codex, its hard to say, and this is what I hope that the codex would do.

It could be a problem with Guilliman himself, thinking of warfare as too straight forward a thing.
Alpharion (or Omegeon) made more complicated and elaborated plans just to prove Guilliman methodical and presise form of warfare inferior to his flexible and multi-facet form.
Heck, when guilliman went to kill his brother, and use feints and suprise withing his plan, Alpharion (or Omegeon) was happy of seing him use such tactics.

As a black templar I don't like it as it has made me and my brothers break up. I don't care if that is heresy in that book. it doesn't mean anything to me.

Halmyr wrote:

The problem is, McNeill's representation of it seems to conflict with everything else we know about it, and he even contradicts himself at times.

Somebody "following the codex" isn't going to be predictable. That's the exact opposite of what the Codex would teach its students to do. It isn't a list of instructions for combat in an "If A, then B" format. The strategy and tactics portion treatise on every conceivable combat scenario, and hundreds of pages of examples and options. Gives the commander on the ground ideas to use in every kind of situation, so they can rely on the experience of thousands of years of warfare in addition to their own initiative. Think about it. Modern military commanders don't have anywhere near that kind of knowledge to draw on, and we've still had military geniuses in the last hundred years. A Space Marine commander well versed in the wisdom of the Codex is like all of them wrapped into one, and more. Sure, the Codex also has boring, regimented stuff like how a Chapter is supposed to be organized, and the way training is supposed to be conducted, and how to manage gene seed reserves, etc. But that's not the bulk of it.

Well, without knowing the full contents of the codex, its hard to say, and this is what I hope that the codex would do.

It could be a problem with Guilliman himself, thinking of warfare as too straight forward a thing.
Alpharion (or Omegeon) made more complicated and elaborated plans just to prove Guilliman methodical and presise form of warfare inferior to his flexible and multi-facet form.
Heck, when guilliman went to kill his brother, and use feints and suprise withing his plan, Alpharion (or Omegeon) was happy of seing him use such tactics.

My problem with that story is that, it's just another poorly written piece of fluff though. The White Dwarf article which contains the story you refer to of the Ultramarines vs the Alpha Legion contradicts itself too. The Alpha Legion knows what the Ultramarines will do because they know the Codex, but then Guilliman takes them by surprise anyway, which seems to suggest that the Codex isn't like that at all.

See, that's the thing, knowing the Codex isn't going to let you predict what your opponent is going to do, because the Codex is going to suggest dozens, if not hundreds, of possible avenues of attack. If both sides know the Codex, they're just going to be extremely talented commanders. Alpharion knew unconventional warfare better than any of the other Primarchs. But Guilliman knew all kinds of warfare better than most people. There wasn't an "inferior" or "superior" way of fighting war. Every battle, every war, every world, is going to need a slightly different approach just like the ways we humans fight now. Really, some of the pidgeon-holed concepts that have been bandied about (like some Chapters being "siege specialists" as if sieging would be the kind of warfare Space Marines would typically engage in) are kinda silly. Space Marines would be specialists in all kinds of warfare, especially if they were Codex adherent chapters. Though, it's fair to say that there are missions that are far more suited to the mentaility and capabilities of Space Marines. To me, siege warfare is a dull, laborious, drawn out affair that Space Marines are going to leave to the Imperial Guard most of the time. They just don't have the interest or numbers for attrition warfare. Look at the depictions of the Space Marines in the "Siege of Vraks" series. More than once, they show up, perform various surgical strikes on specific targets, and then withdraw, leaving the actual "siege" to the DKoK. The same reason I've always thought the idea of the Imperial Fists as "defensive" minded seemed a bit out of place. For Dorn during the Great Crusade, he had an entire legion to work with, and subordinate commands of the Imperial Army. the "modern" Imperial Fists Chapter Master has only a fraction of the manpower and again, defensive patterns aren't the way Space Marines would be properly employed. Every good Space Marine commander and sergeant probably knows hundreds of defensive schemes for use in situations where they demand them, but no Space Marine chapter is going to expend its limited resources concentrating on defense. Again, that's the realm of the Guard and PDF.

I mean, if you say a chapter "Doesn't follow the Codex", that can mean any number of things. For example, the Blood Angels are a "Codex Chapter" for the most part, but certain peculiarities of their gene seed and doctrine has led to variations at the organizational level than the Ultramarines (their Death Company, heavier use of assault units, etc). But they are still probably using the tactical and strategic guidance of the Codex, as well as more or less adhering to the "Ten Company" format. On the other hand, the Space Wolves and Black Templars eschew nearly all of the organizational tenets of the Codex in the way they break down their forces and train their Marines. But do they truly consider everything in the Codex to be garbage? And, even if they do, is that really even a "good" thing? Is being a "Codex Chapter" even a "bad" thing? I mean, there's a reason why the vast majority of Space Marine chapters are "Codex Chapters" and why Space Marines continue to be so effective and successful. There's a lot of knowledge there, even if the Wolf Lords or High Marshals don't want to pay attention to the Codex Astartes prescriptions for the training and organization of a chapter. Seems like it would be kind of foolish to do so.

This is the kind of debate and discussion that can and will go for days and pages if unchecked. It's definitely very obvious that the varied quality and large quantity of Space Marine fluff, and the varied amount of knowledge about warfare of various White Dwarf and Black Library authors bleeds into creating a muddled and confusing picture of Space Marine combat for fans to look at. The lack of a unified and coherent summary of the Codex's tenets doesn't help either. You have to do a lot of reading and research to put together the kind of knowledge I've assembled into this thread (that and my real world experience as a leader of Marines). I'm not bragging, because it shows a real lack from GW. Players shouldn't have to do that much research across two decades and thousands of pages of fluff. The kind of analysis that players have to do of the Codex is silly, because it would take one or two fluff paragraphs in the Codex Space Marines or the core rulebook to eliminate a lot of the confusion, and they have never done it. And I feel like it's out of poor writing, not out of conscious omission.

The Codex itself is undoubtedly an invaluable text, the Imperium's equivalent of Sun Tzu's 'Art of War', but it is sometimes problematic in how it is applied. Those who treat it as an invaluable document and incorporate it into their strategies are made stronger. Those who believe it is the sum total of all military knowledge are going to be weaker for being closed minded.

I find the whole Gulliiman vs Alpharius thing actually quite interesting, because it shows that Gulliman's thinking did show some limitations. Whilst he had an incredible comprehension of conventional warfare and his Codex is undoubtedly a flexible document, there were clearly limitations, or Alpharius would not have contested his view so strongly (I'm basing this off new as well as old fluff, Alpharius is critical, albeit kinder, of Gulliman in 'Legion').

Alpharius taught flexibility and redundancy to a far greater degree than Gulliman did, we know this for certain as it was at the heart of their dispute. Therefore although the Codex is said to contain hundreds of different approaches in any given situation, it must have some limitations. Additionally, we are told that Gulliman did not approve of the Alpha Legion's secretive ways, so whilst he must have had a tactical appreciation of stealth and subterfuge, his honour may have blinded him to some strategies.

In short, the Codex seems to be the greatest military tome ever written, but it is likely based upon a traditional view of warfare that may not work as well under exceptional circumstances. Most Space Marines in most situations could probably follow it's teachings to the letter and win. However there are going to be times when it is less effective than using some serious outside-the-box thinking.

I will paraphrase my reply from another poster on another site I discussed with.

Halmyr wrote:Althought we have no idea was is written in the codex, from what I understand, outlines a aggresive form of combat, lighting warfare.
To use the minimum amount of ressources for the maximum level of effectivess.
Basicly a futuristic form of "art of war" by sun Tzu.

Most chapter look at it with advice but can think beyond it and adapt to situation.
The Codex becomes a problem when it is taken to the word, like the way Graham Mcneil portrais the Ultramarine.
Yes its a reliable way to fight you enemy, but as soon as the enemy knows how you operate, your pretty much done.

But then I would also ask why the Codex wouldn't address the danger of always relying on the same approach repeatedly, or applying an approach that has worked or is suggested for one situation to a situation that might be similar but has some crucial altered parameters.

Originally strict adherence to the Codex was not a bad thing.

McNeill introduced a problem that did not exist in the first place. McNeill wrote them as misguided dogmatists when originally they were not. Originally the Ultramairnes were presented as one of the most successful and famous chapters in the Imperium and they were stated to ridgedly follow the Codex Astartes. One would logically assume Codex=good.

It had been canon since at least 2nd Edition, but probably even since late 1st Edition, that the Ultramarines adhere strictly to the teachings of their Primarch. McNeill is not introducing a change as in the Chapter only just now starting to stick rigidly to his teachings. That is what they have done for the past 10,000 years. It only now dawns on them that this is detrimental, and that the teachings of their Primarch will often steer them wrong. What McNeill has introduced is explicitly the notion that following the Codex suggestions will sometimes be detrimental, and he has demonstrated that with numerous contrived set pieces where the Codex Astartes will give insanely restrictive and broad advice that fails to consider the specific situation the protagonists are facing.

I have no problem with the Ultramarines strictly following the guidelines of the Codex Astartes. What I have a problem with is a sudden explanation that always strictly following the Codex Astartes might be a bad thing, and might cause a Chapter to lose when they could have otherwise won. What I am saying is that when the Codex Astartes makes an explicit suggestion, then that suggestion will be the most beneficial route, but in large parts the Codex will simply not be as specific. You will never have the exact same situation twice, so it would be daft to be overly specific. You will have very similar situations, so you can still learn from the past and make suggestions for future battles. But it is almost impossible to make "allways do X, and nothing else, ever" suggestions.

A Primarch having fought in the Great Crusade for 200 years, then having fought all over the galaxy during the Scouring, then having led a Chapter for the remaining 100 years should have known this. Any Space Marine commander of 50+ years of service should know this. The Codex cannot account for every eventuality and every combination of circumstances immaginable, and therefor can on large parts only deal with generalities or basic situations. It is part of McNeills own premise that the Codex includes specific dictates for generic issues, and/or that the Ultramarines interpete the general suggestions to be as specific. He describes a Codex Astartes to us and sets the Codex up to fail, essentially. The Codex is only needlessly restrictive, or only interpreted in such a manner, where McNeill says it is.

I would find this implausible if the Codex Astartes had been written, and was then inpterpreted, by humans who were supposed to be adapt at military matters. It outright defies my suspension of disbelief if a semi-devine military mastermind would write it in such a way, or if beings with an enhanced intellect, tooled for nothing but a life of war, would interprete it that way.

The lore had always described the Ultramarines as strictly following the Codex doctrines. McNeill has interpreted that as the Codex doctrine being strict.

But as for the issue of a Codex commander being predicted by the enemy……

The Codex’s commander’s approach would naturally be affected by what he has. The Codex would suggest a different approach if the commander had low ammunition, or if the commander had bike instead of assault marines at his disposal. His each approach would be affected by this.

In other words the enemy commander, if he had the Codex, would have to be as familiar with it as the other Codex commander and he would have to have perfect intelligence on the commander’s supplies, composition of forces and preferences. In short the enemy commander would have to know every possible piece of enemy intelligence and variable in order to accurately predict the Codex commander’s actions.

And in that case it would not matter of the Astarts force followed the Codex or not.

Halmyr wrote:
Well, without knowing the full contents of the codex, its hard to say, and this is what I hope that the codex would do.

It could be a problem with Guilliman himself, thinking of warfare as too straight forward a thing.
Alpharion (or Omegeon) made more complicated and elaborated plans just to prove Guilliman methodical and presise form of warfare inferior to his flexible and multi-facet form.
Heck, when Guilliman went to kill his brother, and use feints and suprise withing his plan, Alpharion (or Omegeon) was happy of seing him use such tactics.

Alpharius takes issue with Guilliman’s tactics because they are inflexible and lack initiative.

Curiously the Index Astartes: Imperial Fists state that the Codex Astartes encourages initiative and flexibility, the Ultramarines article states that the battle company is intended to be a flexible formation and the Deathwatch books talk about his the Ultramarines prize fluid tactics and adaptability.

wulfmojo wrote:
In short, the Codex seems to be the greatest military tome ever written, but it is likely based upon a traditional view of warfare that may not work as well under exceptional circumstances. Most Space Marines in most situations could probably follow it's teachings to the letter and win. However there are going to be times when it is less effective than using some serious outside-the-box thinking.

The thing is in McNeill’s stories Ultramarines cannot go outside the Codex. Originally in the Second Edition fluff they were merely described as strictly adhering to the Codex. You can still strictly adhere to something and still do other things.

At one point in the Ultramarines books Uriel Ventris comes up in a situation where the Codex does not cover improvised explosives, a very basic military technique. The fact that the Codex does not cover basic military techniques is problematic, but Uriel treats like inventing a new solution as something completely new and heretical, as does the other Ultramarines.
So in McNeill’s view, if they come up to a situation that the Codex simply does not cover, the Ultramarines can’t act outside the Codex. It’s a wonder how McNeill’s Ultramarines survive the Tyranids, Necrons or Tau at all.

That's the ultimate problem with McNeill's books on the Ultramarines, and this lack of broader thinking shows through painfully in some of his other books. The Codex cannot be both a work of military genius and a work of military buffoonery, and yet it alternates between the two constantly in the stories I've read. I agree that there will be limitations. Primarch or not, it's reasonable to assume Guilliman's work wasn't perfect, but when you have Marines quoting the guy as saying that the Codex was meant to be a guideline and not a replacement for initiative, and then you have Ultramarines doing the exact opposite (or offering surprised praise when that advice is actually followed), it' just a sign of bad authorship. The Ultramarines could not be the "greatest of Space Marine Chapters" if what makes them the best is also what limits them the most. Contrived situations that ruin suspension of disbelief indeed.

Let's just be realistic, Graham McNeill doesn't write Space Marines well at all. The only strong characters in his Horus Heresy novels have been the Remembrancers. The normal humans.

Well, since people are talking about McNeill, I'm guessing it's about Ventris' opinions on the codex in Ultramarines?

It seems to me, that, following the story of Age of Darkness.

Gulliman originally wrote the Codex quite prescriptively (If X happens, do Y) as an experiment. He ordered some wargames to try it out, which failed at the end. So, he decided to rewrite it to be more generalised and sort of more advice based.

And generally speaking, that pretty much worked fine.
But, add 10000 years, and I can imagine, adding new additions to it, a revision or two Bible style and well, maybe it's become a little more like the former than the latter.

However, what if we then get different kinds of Ultramarines? The 'good' ones, who know it's supposed to be a tool for advice and the 'bad,' unimaginative ones to do treat it like a holy scripture. - This point is actually made in the Space Marine game.

I follow the views of Dorn, Russ and Vulkan. It is useless, if somebody has the ability to maintain his 10,000 strong legion, especially the loyalist primarchs who had PROVEN themselves time and time again in the Heresy, let sleeping dogs lie.

Again, I cannot stress this enought, since we do not know what is written in the codex, its hard to know.

I'l take an example: When Magnus is Assaulting Honsou warship in "the Chapters due", Magnus orders for the warship to spread because they are entering a debris/mine field.
Not a bad decision, give ships more room to move around and avoid debris, plus if some one gets hit, it leaves a nice space for the other. The problem came because
Honsou knew how he would react and used it against them to great effect.

But they can think outside the codex, look at Sicarius stealing a enemy rhino and sneaking behind enemy line for a assasination attempt (same book as first example)

The codex seems to be somewhere between "this is how you a fight a war" and "heres a few suggestion to help you"
More of a "here are the best way to fight when theses situation arise"
But I'm sure, and hope, the Codex a warning printed in small text "these are example, please feel free to use your own jugment depending on battlefield situation"