The problem I always had with uncrittable creatures was that so many of them seemed like they really should be crittable.
Corporeal undead? Why not? Everyone knows you shoot zombies in the head. Incorporeal undead are tougher, but if you can hurt them at all, why shouldn't stabbing them in the face be extra effective?
Golems? Well, this changes depending on how you imagine golems. If you imagine them as big rocks formed into a human and walking around, then crits don't make much sense. If you imagine them as having bolts and couplings and structure, then it does. I've always preferred my golems to look like someone built them, but that's a subjective thing.
The only monsters I can't see the logic of permitting crits on in any way would be oozes.