In misery they are sponges, but one shot to the head/spine and they go down, so that's easy enough. The only real bullet sponge NPCs you come across are the super zombies or whatever, in the base game people would farm them easily for good gear. That paired with bullet sponges as you can imagine is not good design, but MISERY has tried to steer away from that as much as possible. Misery to my experiences is about putting an emphasis on the survival mechanics, making them more difficult, making ammo scarce and useful, more realistic guns, etc. The 'Complete' mods make enemies into bullet sponges though. Mutants that generally have quite a bit of health are mutants like the pseudogiant.

Obviously though mercenaries and stuff sometimes wear exoskeleton armour which is endgame gear nonetheless, so they don't die in 1 hit but take a couple of bullets, but head shots always insure rapid kills. I too don't enjoy bullet sponges in FPS games, but MISERY has more advanced AI, mobs are just generally more difficult to deal with.

Stalker cop misery mod