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gabrielhorus wrote:The Tau would be more than able to hold off the Tyranids. The Tau Custodian class capitol ships can obliterate an Imperial fleet without backup. The Tau have four to unleash against invaders and they are building more.

Most Bioships would die to these vessels before they could release spores. The Tau response to landings would be a defensive line and a strategy I like to call "It's Rainin' Drones".

Now to move into more serious things, no. The Tau do have nice gadgets and weapons, yes. But they could not have any hope in defeating a major hive fleet. The Imperium of Man is hundreds of times larger than the Tau Empire, and it nearly failed to stop the Hive Fleet Leviathan. The Tau can destroy a splinter fleet. And have already done so.

They simply lack the manpower to even give the Fleet a challenge. Besides, when the Hive Mind discovers that the Ethereals are guiding the others with hormones, they would strike at them first. Without them, the Tau would panic and be so disorganised that their Empire would fall in weeks, if not days. The rule "Shoot the big ones!" also applies to the Tau: "Shoot the Ethereals!"

The tau in theory could and should be using their automation to fight in place of organic troops, unless there is some overriding reason (doctrine, fear, problems with technology, etc.) preventing them from doing so. Their fighters, tanks, battlesuits, etc could and probably should all be droid controlled or remotely piloted (They've at least shown the capability exists) but.. for whatever reason.. they don't. Or if they needed squishy organics they could just let their auxiliaries fight inside the suits (with Tau crews controlling andb acking up the systems - there's plenty of ways they can work safeguards in ot prevent that tech being used against them.) But they don't, nor do they Drone-spam anyone in place of conventional forces, so there may be some obvious limitations of some kind (or they just are incapable of thinking of it so far - they have shown quite a bit of naitvete on the galactic scale.)

As to whether they can survive a hive fleet.. it depend entirely on how big a hive fleet we are talking about. A small one (like they faced in the Tyranid Codex or like Behemoth) probably. The more usual "million ship/billions of tyranid" type force.. no. They may be able to destroy it cetainly, but destroying and surviving are two entirely different things. The Tau are, territory wise, as large as (maybe a big larger) as an Imperial Sector in terms of worlds (somewhere in the hundreds of inhabited worlds.) They have some technological advantage,s but their main asset is solid logistics and a standardized kit/gear amongst the tau-derived troops (which your typical Guardsmen does not have. Hell even lasguns and flak aren't guaranteed nowadays.) On the other hand, they typically have smaller populations per world than the Imperium does (A major Sept world has a fraction of the population of a hive world) And the Imperium can generally expect to lose scores if not hundreds of worlds stopping a sizeable hive fleet (EG Kraken) that means the Tau could expect to lose a fair chunk of their empire, if nto a majority of it, stopping the Tyranids.

The small size of the Tau, their generally low signature in the Warp, and their low population densities are likely what keep them from being noticed (compared to the Orks, Eldar, Humans, etc...)

agnosto wrote:
Not to split hairs but it's their drone technology that's enabled them to expand as quickly as they have. From Hammerheads on up to Tau and starships (Manta have 16 drone controlled turrets), Tau vehicles have fully integrated drone technology. The fluff talks about automated factories which enable them to quickly adapt craft to their needs (BFG) and the codex has an excerpt from an imperial source that talks about advanced production and agriculture tech that meets or ::gasp:: exceeds IoM tech.

To an extent. My point was that they can't just send out millions of Drones to fight wars for them. They need boots on the ground, and in the tanks, and in their starships.

The Tau have gone through 3 phases of massive expansion and could not have accomplished this without drones since the number of Tau is relatively small. IA:3 talks about how extensively drones are used and even the codex allows for units for drones without a controller. IA:3 has heavy gun drones that may be taken in units of 2-6 as their integrated FoF tehcnology and preprogramming allow them to accomplish basic missions without a controller; there's nothing more basic than "Go here and kill any enemy along the way." There are other types of drones like sentry turrets and tech drones as well.

If they could use Drones to simply out-last a Tyranid Hive Fleet, then they'd have expanded far more than they currently have. That's my point. They have Drones in large numbers, but those numbers are still no way near large enough to starve the Tyranids of biomass.

1.) What happens when humans invade a Tau home planet? Tau first evacuate the whole civil population then start a fight (see novel "White Scars"), but retreat if necessary. Tau are not territorial at all, not even on their homeplanet. So if a big Hive Fleet approaches, Tau would just avoid them, not run into close combat to prove their manhood.
2.) Concerning the drone rebellion: Not every intelligent life form starts with killing its creator. Not every child tries to kill its parents to get free of an unequal relationship. Actually most children appreciate their parents.

SomeRandomEvilGuy wrote:

agnosto wrote:
Not to split hairs but it's their drone technology that's enabled them to expand as quickly as they have. From Hammerheads on up to Tau and starships (Manta have 16 drone controlled turrets), Tau vehicles have fully integrated drone technology. The fluff talks about automated factories which enable them to quickly adapt craft to their needs (BFG) and the codex has an excerpt from an imperial source that talks about advanced production and agriculture tech that meets or ::gasp:: exceeds IoM tech.

To an extent. My point was that they can't just send out millions of Drones to fight wars for them. They need boots on the ground, and in the tanks, and in their starships.

The Tau have gone through 3 phases of massive expansion and could not have accomplished this without drones since the number of Tau is relatively small. IA:3 talks about how extensively drones are used and even the codex allows for units for drones without a controller. IA:3 has heavy gun drones that may be taken in units of 2-6 as their integrated FoF tehcnology and preprogramming allow them to accomplish basic missions without a controller; there's nothing more basic than "Go here and kill any enemy along the way." There are other types of drones like sentry turrets and tech drones as well.

If they could use Drones to simply out-last a Tyranid Hive Fleet, then they'd have expanded far more than they currently have. That's my point. They have Drones in large numbers, but those numbers are still no way near large enough to starve the Tyranids of biomass.

I think they could deploy them in large enough numbers to starve the nids. Pound per pound a tau army is very light on bio mass. Tau deploy in smaller armies and are very mechanized. That is before they focus on drones. They could deploy lines of drone maned turrets, deploy squads of gun drones to flank, then snipe snaps creatures with sniper drones. The only place they need people is in the command ranks. Now that they know about the nid threat, you can bet they have a few of these anti nid fighting forces around.

On the other hand, they would be defending a populated planet and civilians make good biomass.

Kroothawk wrote:1.) What happens when humans invade a Tau home planet? Tau first evacuate the whole civil population then start a fight (see novel "White Scars"), but retreat if necessary. Tau are not territorial at all, not even on their homeplanet. So if a big Hive Fleet approaches, Tau would just avoid them, not run into close combat to prove their manhood.
2.) Concerning the drone rebellion: Not every intelligent life form starts with killing its creator. Not every child tries to kill its parents to get free of an unequal relationship. Actually most children appreciate their parents.

Avoiding a hive fleet is the worst way to handle one. Assuming it dosen't follow you, it has time to murder the bio sphere of your planets and grow unrestricted. The tau sit on some very prime planets. A dense cluster of habitable planets and the nids would turn them into rocks. The tau would never be able to recover.

No, probably not. I think i remember a thread on here that said it would take .04% of IG forces to wipe out the tau. If it takes that little to kill them from an imperium standpoint, I cant seem them surviving a hive fleet. Its hard to the Imperium and other more formidable Xenos to do that.

1hadhq wrote:

TrollPie wrote:


And people who think Tau drones will all suddenly rebel and destroy their empire are forgetting this has happened once in the same setting because at the time Terminator was pretty popular, and that well programmed computers don't rebel.


Also, there are about 30 fully developed planets in the Tau Empire and 70 minor planets or allied worlds. The total population of the Tau is most likely in the hundreds of billions. A ridiculously large proportion of that is in the Fire Caste. It's nothing compared to the Imperium, but it's density and Caste system means it's far from an easy meal.
.

Crons still be "terminator" to some degree, so we'll see what happens when they return...

The total population of the Tau is only shown in ....their first dex... and probably much lower than some fans want it to be.
Just 100 planets? high chance the nids eat the important ones and the "empire" crumbles in a few months. You don't need to annex every place in an "empire" to end said "empire".

IIRC the Eldar weren't used to save the Tau, so there is still hope to survive the nids by the grace of the 'elder' races....

The population of 20 Earths is roughly 140 billion. The Tau have thirty well developed worlds and many small ones. Therefore the Tau Empire could easily number in the hundreds of billions, especially given their industrialised nature.
Tyranids eat the important planets and then the Empire crumbles? Well, since the Tau Empire's most important planets are all 1st Phase Sept worlds which are heavily industrialised and produce a large proportion of the Fire Caste, this will be no easy feat. The complete lack of importance that the Tau place on territory means that if they needed they would evacuate all of their endangered worlds and send their leaders to safer places anyway. Simply destroying their important planets isn't enough; you have to destroy every safe haven they have to ensure their leaders don't escape. Not to mention the Tyranids would have no way of knowing which Tau worlds are important anyway.
Also, stop putting the word Empire in inverted commas. Name one known Empire in 40k outside the Warp bigger than the Tau. I can only think of one.


Automatically Appended Next Post:

SomeRandomEvilGuy wrote:
To an extent. My point was that they can't just send out millions of Drones to fight wars for them. They need boots on the ground, and in the tanks, and in their starships.

On the ground? Seeing as drones feel no fear or pain, will always act rationally regardless of how much danger they're in, have far more accurate weapons than an unspecialised living organism and can move at far greater speeds over any terrain than regular troops (yay frizbees!) they're pretty much perfect combat troops.
In tanks? See above.
In their starships? Drones already control their Starships. A living creature can't target things millions of kilometres away moving at speeds inconcievable to human minds. They also can't manouvre ships at these speeds, or work out where themeselves and the enemy are, without the aid of computers. Swap the living creatures for repair drones and strategising computers and you have a fully combat capable starship.

nomotog wrote:
I think they could deploy them in large enough numbers to starve the nids. Pound per pound a tau army is very light on bio mass. Tau deploy in smaller armies and are very mechanized. That is before they focus on drones. They could deploy lines of drone maned turrets, deploy squads of gun drones to flank, then snipe snaps creatures with sniper drones. The only place they need people is in the command ranks. Now that they know about the nid threat, you can bet they have a few of these anti nid fighting forces around.

Unfortunately, the Tau army has to been on a planet. They have to hold it and stop the Tyranids from consuming it. That doesn't allow you to just use smaller armies. We know the Tyranids can use amazing tactics, and we know that gun lines end up being destroyed by whatever bizarre creature the Tyranids come up with. You could have dozens of lines of Drone turrets and it would only delay them and the Tyranids would recover more quickly than the Tau. They need sufficient people to command the Drones to attempt to out-think the Tyranids. That alone will require large numbers of Tau. Drone turrets would be far too inflexible to prove much of a hassle. For one thing, they're defenceless against burrowing creatures.

On the other hand, they would be defending a populated planet and civilians make good biomass.

Not to mention the planet itself. All the plants and such as well as the actual minerals and water.

Billagio wrote:I think i remember a thread on here that said it would take .04% of IG forces to wipe out the tau.

You shouldn't really take everything said on Dakka Dakka as gospel truth.

TrollPie wrote:Name one known Empire in 40k outside the Warp bigger than the Tau.

Doesn't it say that the Tau on the same scale as dozens of other xeno empires?

On the ground? Seeing as drones feel no fear or pain, will always act rationally regardless of how much danger they're in, have far more accurate weapons than an unspecialised living organism and can move at far greater speeds over any terrain than regular troops (yay frizbees!) they're pretty much perfect combat troops.

Then why aren't they used as the perfect combat troops? Why do the Tau have to use actual Tau on the ground? Why aren't they invading vast swathes of the Imperium instead having to fight hard battles or negotiate with Imperials?

In their starships? Drones already control their Starships. A living creature can't target things millions of kilometres away moving at speeds inconcievable to human minds. They also can't manouvre ships at these speeds, or work out where themeselves and the enemy are, without the aid of computers. Swap the living creatures for repair drones and strategising computers and you have a fully combat capable starship.

They still require the Air Caste for a reason. Drones are not the be all and end all of Tau warfare.

SomeRandomEvilGuy wrote:

TrollPie wrote:Name one known Empire in 40k outside the Warp bigger than the Tau.

Doesn't it say that the Tau on the same scale as dozens of other xeno empires?

TrollPie wrote:

The fact that they are unnamed but on the same scale doesn't support your position at all.

SomeRandomEvilGuy wrote:

TrollPie wrote:

The fact that they are unnamed but on the same scale doesn't support your position at all.

It was just a little joke. But seriously, they're pretty much the same size as almost any other Empire in 40k so when someone keeps putting the word "Empire" in inverted commas it's pretty annoying.

Depends on how you class an empire. Every other major race is more populace, the Tyranids, the Imperium, the Orkz, the Necrons, and arguably the Dark Eldar and Eldar. The former four are probably thousands of times more numerous. It's doubtless that there are other races out there that could number close to the Tau, spread out mind you. But I digress, the Tau are the second largest unified faction. Oh no, maybe not, because Abaddon and Huron rule their own sizeable areas of space. Huron we know controls many systems, and billions of lives.

SomeRandomEvilGuy wrote:

On the ground? Seeing as drones feel no fear or pain, will always act rationally regardless of how much danger they're in, have far more accurate weapons than an unspecialised living organism and can move at far greater speeds over any terrain than regular troops (yay frizbees!) they're pretty much perfect combat troops.

Then why aren't they used as the perfect combat troops? Why do the Tau have to use actual Tau on the ground? Why aren't they invading vast swathes of the Imperium instead having to fight hard battles or negotiate with Imperials?

Because they wouldn't sell as well without actual Tau models.

In their starships? Drones already control their Starships. A living creature can't target things millions of kilometres away moving at speeds inconcievable to human minds. They also can't manouvre ships at these speeds, or work out where themeselves and the enemy are, without the aid of computers. Swap the living creatures for repair drones and strategising computers and you have a fully combat capable starship.

They still require the Air Caste for a reason. Drones are not the be all and end all of Tau warfare.

And what reason would that be? Maybe they're just on standbye in case of a malfunction. Maybe so that they can add a little initiative. But against Tyranids, they could easy be replaced by less juicy meals and combat capability would barely drop at all-it would probably increase.

SomeRandomEvilGuy wrote:

nomotog wrote:
I think they could deploy them in large enough numbers to starve the nids. Pound per pound a tau army is very light on bio mass. Tau deploy in smaller armies and are very mechanized. That is before they focus on drones. They could deploy lines of drone maned turrets, deploy squads of gun drones to flank, then snipe snaps creatures with sniper drones. The only place they need people is in the command ranks. Now that they know about the nid threat, you can bet they have a few of these anti nid fighting forces around.

Unfortunately, the Tau army has to been on a planet. They have to hold it and stop the Tyranids from consuming it. That doesn't allow you to just use smaller armies. We know the Tyranids can use amazing tactics, and we know that gun lines end up being destroyed by whatever bizarre creature the Tyranids come up with. You could have dozens of lines of Drone turrets and it would only delay them and the Tyranids would recover more quickly than the Tau. They need sufficient people to command the Drones to attempt to out-think the Tyranids. That alone will require large numbers of Tau. Drone turrets would be far too inflexible to prove much of a hassle. For one thing, they're defenceless against burrowing creatures.

What I meant by small is that a tau army is about 100-200 people and it flattens the ground that would take other sides 500-1000 people. The tau need to deploy far fewer people to combat. They would still use a lot of people, but no where close to the number of people used by other armies.

Can nids recover from a combat where they gain no biomass? I mean if a squad of nids gets flatted by a drone force, they lose biomass. If the nids flatten the drone force, they get no biomass. It dosen't just delay them it weakens them.

I don't know how many people you would need to command a drone army. Not many though. Drones can be deployed in squads without tau to command them. Same with the drone turrets. (I am thinking about the FW turrets here not some hypothetical turret.) You would still want crisis suits tanks and other tau maned vehicles, but the amount of biomass you get from cracking open a tank is less then the amount that the tank shoots up.

Just the fact that the tau army is mostly robots makes them a poor target.

The important thing to note is a Tau army would have an easier time killing a Synapse creature than an Imperial Army with their massive range.

Also they would just throw Kroot and Vespids in to tie up the waves of gaunts and stealers while the Tau themselves would fire upon Monstrous Creatures.

Kilkrazy wrote:It's pretty clear the Tyranids will wipe out all life in the galaxy. They've already done it to a different galaxy.

It's been posited that, because of the various directions that the Hive Fleets have entered the galaxy, they may have each come from a different galaxy, and the true Tyranid force has consumed the vast majority of the local cluster, meaning that it is possible that out of four or five galaxies, the Tyranids have consumed them all, and we are the last ones left.

The Tau could not stand against the full force of a Hive Fleet.

The Orks are probably the best bet against permanently staving off the Tyranids. In the Ork codex (3rd ed, pg 33) it states how there are tens of millions of more Orks in one of the Segmentums, and that when it comes to the Orks, there is literally strength in numbers (more Orks = stronger Orks).

It also states if all Orks united under one banner the combined might of all other races couldn't stave them (they don't exclude Tyranids so they must also be included).

The problem is they would never do it.

nomotog wrote:

SomeRandomEvilGuy wrote:

nomotog wrote:
I think they could deploy them in large enough numbers to starve the nids. Pound per pound a tau army is very light on bio mass. Tau deploy in smaller armies and are very mechanized. That is before they focus on drones. They could deploy lines of drone maned turrets, deploy squads of gun drones to flank, then snipe snaps creatures with sniper drones. The only place they need people is in the command ranks. Now that they know about the nid threat, you can bet they have a few of these anti nid fighting forces around.

Unfortunately, the Tau army has to been on a planet. They have to hold it and stop the Tyranids from consuming it. That doesn't allow you to just use smaller armies. We know the Tyranids can use amazing tactics, and we know that gun lines end up being destroyed by whatever bizarre creature the Tyranids come up with. You could have dozens of lines of Drone turrets and it would only delay them and the Tyranids would recover more quickly than the Tau. They need sufficient people to command the Drones to attempt to out-think the Tyranids. That alone will require large numbers of Tau. Drone turrets would be far too inflexible to prove much of a hassle. For one thing, they're defenceless against burrowing creatures.

What I meant by small is that a tau army is about 100-200 people and it flattens the ground that would take other sides 500-1000 people. The tau need to deploy far fewer people to combat. They would still use a lot of people, but no where close to the number of people used by other armies.

Can nids recover from a combat where they gain no biomass? I mean if a squad of nids gets flatted by a drone force, they lose biomass. If the nids flatten the drone force, they get no biomass. It dosen't just delay them it weakens them.

I don't know how many people you would need to command a drone army. Not many though. Drones can be deployed in squads without tau to command them. Same with the drone turrets. (I am thinking about the FW turrets here not some hypothetical turret.) You would still want crisis suits tanks and other tau maned vehicles, but the amount of biomass you get from cracking open a tank is less then the amount that the tank shoots up.

Just the fact that the tau army is mostly robots makes them a poor target.

When has a Tau army ever been "mostly robots"? They use a significant amount of Drones, but you never see them numbering more than Firewarriors or Kroot. They don't pilot vehicles, there's still a Tau inside every mech and tank. Face it, they simply don't have the numbers or the resources. The Imperium defeats the Tyranids because they have the brute strength to fight the wars of attrition that Tyranid engagements inevitably become. The Tau can't engage in this kind of battle, their doctrines are designed to make them avoid such battles.

Mostly robots is a poor way to put it. There army is mostly machine. That is pound to pound you get more inorganic matter then organic matter.

It depends on exactly how the hive fleet works, but a tau planet will fair better then a IoM planet.

It's because Tau armies are generally smaller, not because they're mostly machine.
And no, a Tau planet won't. Tau aren't noted for entrenching themselves, and wars of attrition are how Tyranid engagements are fought. They would fair much, much worse.

iproxtaco wrote:It's because Tau armies are generally smaller, not because they're mostly machine.
And no, a Tau planet won't. Tau aren't noted for entrenching themselves, and wars of attrition are how Tyranid engagements are fought. They would fair much, much worse.

They are smaller and mostly mechanized. Every element in the tau (except kroot vespin) is a mechanized element.

Then there is two things. You don't actually have to engage in an attrition war with the nids. Actually such a war is the worst way to fight the nids. You will slowly grow weaker well they get stronger. To stop a nid fleet, you need to be fast deadly taking them out in a killing blow.

The next thing, even though the tau codex says that the tau don't defend land or entrench. In stories, the tau are almost always entrenched. It's a disconnect between what they say and what they do. What version is right? Bugger if I know.

Tau Drones require controllers.

In order for Drones to be on the ground fighting, the Fire Caste must be on the ground as well operating with Drone Controllers.

Kanluwen wrote:Tau Drones require controllers.

In order for Drones to be on the ground fighting, the Fire Caste must be on the ground as well operating with Drone Controllers.

Not always. When operating in large numbers Drones form a sort of network intelligence, that's why Gun Drone squads have no controllers.

TrollPie wrote:

Kanluwen wrote:Tau Drones require controllers.

In order for Drones to be on the ground fighting, the Fire Caste must be on the ground as well operating with Drone Controllers.

Not always. When operating in large numbers Drones form a sort of network intelligence, that's why Gun Drone squads have no controllers.

And when they form those "networked intelligences", Drones behave in a far different fashion where they'll fall back or 'break'.

Kanluwen wrote:

TrollPie wrote:

Kanluwen wrote:Tau Drones require controllers.

In order for Drones to be on the ground fighting, the Fire Caste must be on the ground as well operating with Drone Controllers.

Not always. When operating in large numbers Drones form a sort of network intelligence, that's why Gun Drone squads have no controllers.

And when they form those "networked intelligences", Drones behave in a far different fashion where they'll fall back or 'break'.

But not out of fear or cowardice, but because they know that they would serve more use if they fall back. A good soldier knows when to retreat.

Who knows, with conventional weapons against an very large fleet probaly not but..

Somewhere underground in an Tau command bunker:

- Now look.. if we don't deal with this now its only a matter of time before the Nids turn our major septs into biogoo.

- So what do you suggest?

- Its simple we ah reactivate the Leviathan project.

- You're crazy.

- No.. I'm not...

( Yeah an ripoff but who cares? )

TrollPie wrote:
But not out of fear or cowardice, but because they know that they would serve more use if they fall back. A good soldier knows when to retreat.

And retreating before the Tyranids is not actually a good idea.

There's a reason the Imperium holds the line as long as they can, and it's not pure stubbornness.

nomotog wrote:

iproxtaco wrote:It's because Tau armies are generally smaller, not because they're mostly machine.
And no, a Tau planet won't. Tau aren't noted for entrenching themselves, and wars of attrition are how Tyranid engagements are fought. They would fair much, much worse.

They are smaller and mostly mechanized. Every element in the tau (except kroot vespin) is a mechanized element.

Which generally means lot of small squads, and lots of vehicle pilots. The Tau's mechanization has little to do with it. Their forces are generally smaller, that's why.

Then there is two things. You don't actually have to engage in an attrition war with the nids. Actually such a war is the worst way to fight the nids. You will slowly grow weaker well they get stronger. To stop a nid fleet, you need to be fast deadly taking them out in a killing blow.

Oh yes, it is. You have to be strong, and hold the line. You can't rely solely on this, Tarsis Ultra is evidence enough, but a Tyranid swarm numbering in the tens of millions isn't going to be taken down by guerilla tactics and mechanized infantry.

[/]The next thing, even though the tau codex says that the tau don't defend land or entrench. In stories, the tau are almost always entrenched. It's a disconnect between what they say and what they do. What version is right? Bugger if I know.

Kanluwen wrote:

TrollPie wrote:
But not out of fear or cowardice, but because they know that they would serve more use if they fall back. A good soldier knows when to retreat.

And retreating before the Tyranids is not actually a good idea.

There's a reason the Imperium holds the line as long as they can, and it's not pure stubbornness.

Programme them to be more reluctant to retreat. That's the advantage of Drones- they'll reliably follow any orders they're given.

The Tau as a force are optimized for conquest and occupation throughout their society (must expand the Greater good and all). This means they are better at taking on the Imperium (especially since their distance from the Imperium means that any sort of coherent or well-supplied response is difficult to amass, especially in the midst of two major ongoing conflicts and quite a few medium and countless minor ones.) which means they have considerable logistical and "home field" advantages compared to the Imperium. Their method of waging war, however, is not well suited to fighting the Orks or Tyranids for the simple reason that the motivations of both groups do not mesh well with a conquest-oriented force. The Orks fight for the sake of fighting and for their own evolutionary development, and the tau literally would have no chance of conquering them. Crushing or destroying them, yes, but that won't end things (since it will just draw more orks. and once they're on the ground they're impossible to purge.)

The Tyranids are another race they simply cannot conquer and will never co-exist with, but their tactics emphasize extreme mobility, even to the point of (as much as they are able to at least) giving up ground rather than holding it. To an extent this makes perfect sense, as mobility is a considerable advnatage (Allowing you to concentrate fire, especially the firepower-heavy nature of the Tau) but that only works so long as you have a solid logistical tail and a strong base to operate from (Factories, recruits, food, etc.) And that means you can only back up so far without giving up your ability to engage in war. The tyranids, as a force, are optimized for attrition warfare (they'll just destroy their troops at the end and rebuild them once they win, so losses are functionally irrelevant. Hell even if they lose they'll keep rebuilding so long as there is avialable biomass.) They can deploy effectively from orbit unless you deny them the high ground (space fleets or strong anti-ship defenses are a necessity) and they engage in rampant and insidious biowarfare (spore mins, corrupting/tyranoforming the planet around the defenders, etc.) The probably numerical superiority (at least per planet, if not parity) is only the icing on the cake.

It also doesn't help that they really don't have that much experience of fighting or knowledge of the Tyranids. Hell, they hardly knew much about Genestealers.