I am generally supportive of the concept, but it needs to be done carefully
IMO. There are two big pitfalls to be aware of:
1. Differing resolution systems - the
40k implementation where damage resolved differently vs vehicles than vs everything else in the game is a poor approach. It requires players to learn basically two parallel damage resolution systems that each had their own separate peculiarities and caveats that could have sometimes wierd interplay with other rules. In turn, it required the design studio to often add additional clauses to other rules to account for how rules or abilities might interact around these two systems, which resulted in additional rules overhead and system bloat. Another issue on the design side was where do you draw the line between them? I.E. why are Dreadnoughts vehicles but not Crisis Suits or Wraithlords? Why are Carnifexes monstrous creatures but not Soul Grinders? A side effect of all this was that certain weapons which could devestate a vehicle in a single shot could barely tickle monstrous creatures (e.g. - a Hammerhead mounted railgun), which resulted in a meta that preferred spamming S6-S8 weapons, as many of these intermediate strength usually fired multiple shots (whereas S9-10 weapons usually only fired 1 shot at most and thus were often considered to be overpriced) which made them more effective in the anti-monster role while also being reasonably effective against vehicles as well.
All-in-all, its a very inefficient approach instead of utilizing a common approach to damage resolution for both types of targets. An example of how this could be better achieved within the
40k framework - use higher toughnesses instead of armor values for vehicles, but larger models have "facings" so they might be more tough when attacked from the front than the rear, etc. and the to-wound chart allows for low strength weapons to be unable to wound high toughness models. Another alternative would be to separte weapon strength into an "anti-personnel" and "anti-materiel" stat, which would allow for weapons that are more effective at killing vehicles but less effective at killing infantry, and vice versa.
2. Invulnerability Traps - the old
40k implementation, in certain matchups under certain conditions, made it possible for certain units to become essentially immune to attack. If I brought a land raider to the table, and you didn't bring any weapons above strength 7 - or if you brought an insufficient quantity of them and I happened to remove them from the table early on - then the land raider was essentially immune to attack as you had no other means to damage it.
While some would argue this to be realistic (and even thats debatable, as battlefield are littered with the discarded weapons of fallen soldiers, and even lightly armed infantry can find ways to knock a tank out of service in a close-up assault using grenades and improvised explosives), I would argue that its poor form from a design standpoint.
This type of play should be rewarded (if you succeed in taking out all your opponents heavy weapons) or punished (if you allow your opponent to destroy them all), but not by rendering units wholly immune to recourse (and by extension games potentially unlosable/unwinnable as a result). If you implement a system in which certain weapons cannot harm certain targets, then I will argue that the game needs to include a system that ensures that a player will always have access to some form of recourse against said target, even if they would lack the "normal" means to do so. This form of recourse should be inefficient - remember, you should be punished for allowing your opponent to snipe out all your heavy weapons and neutralize your most capable weapons, but that doesn't mean you should just have to take the worst of what your opponent has to offer with no means to strike back. In other words, your game needs a sort of "catch-up mechanic" that allows for a player to make sub-optimal or higher risk/low reward strikes against targets which are otherwise immune to the effects of the remaining weaponry on the table.
There are numerous ways to achieve this and justify it, in the context of older editions of
40k they did so by way of grenades. Most units in the game had access to some form of them and their use in the assault phase allowed for even baically equipped light infantry units to harm most vehicles under certain circumstances - though I note that there were some armies that basically had little/no options in this regard, so somewhat poorly implemented in that sense. In a more modern iteration of
40k where the to-wound chart didn't allow for any weapon to harm any target on a 6+, I would argue for the inclusion of an "improvised weapon" strategem, one model gets to make an attack with like Heavy 1 R:12" S:8
AP:-1 D
6 or something like that. Not enough to really be a major game changer, but still enough that a player won't be totally helpless. Another example is to borrow a page from Warmachine and institute a combined attack option (potentially also as a strategem) - nominate a model in a unit to make an attack, then you can nominate a number of additional models in the same unit to trade their own attacks in order to support the attack being made by that mode - each model that supports that attack adds +1S and +1D to the attack up to a max of +X/+Y, or something (in melee this could be justified by the models helping pull access panels off a tank in order to plant grenades in more vulnerable areas, for a ranged attack it could be explained as the models coming across an abandoned crew-serve weapon and the group coming together to load, target, aim, and fire an artillery round or whatever at the opposing vehicle).
