Honestly, I think the only creatures that should be immune or highly resistant to precision damage and crits are incorporeal creatures and gaseous creatures. You can hit an ooze or creature made of water in a thinner portion of their body and send more of whatever composes their body flying, or put out more of the fire of a fire elemental with a particularly forceful swing. You can smash skeletons particularly hard just like you can smash humanoids with skeletons particularly hard, and you can find a weak spot in an earth elemental or stone golem just like miners can find weak spots in regular rock formations. And of course plants are actually alive, with sap and stems and other sorts of critical areas, so they should obviously be crittable.

This isn't to say that some creatures shouldn't be resistant to precision damage or crits, but it should be relatively rare and shouldn't be done on a type-wide basis, or once again you have the issue of having the same crittability factor for ghosts and skeletons and everything in between. The idea that you can't possibly find any weaker-than-normal point whatsoever is ridiculous unless you're talking about something that's not solid or liquid, something that's not even physically there, or a perfectly spherical perfectly uniform object...and as the latter don't tend to come up much in D&D, it's only the ghosts and air elementals that we really need to worry about.