Abominations should be ungrabbable.

[GARGATHUGANOKSREVENG]

What's stopping the abomination from just punching the guy grabbing him. I don't think you should remove features from the game

Calling that "removal of a feature" is a very odd take. We're modifying a feature here, not removing it.

And giving some mobs, like abomination-form,  grab-immunity for balance reasons is very sensible.

Grabbing is a feature. It allows you to do a lot of things. Getting rid of the ability to grab abominations is getting rid of that feature, in that specific area. It removes a whole lot of potential ingame, just because a handful of players don't want to put effort into fighting.

Making the grab harm you, however? I don't think players should be punished for using one of their only strong abilities against an incredibly powerful antagonist but it's not as bad as removing the ability to grab altogether. 

The player hitbox is pretty small even when you're a regular human being grabbed by another human. It's a great thing that people, including shambling abominations*, can just press the resist key to break out of grabs. 

*I haven't tested it out myself but I don't see why it wouldn't work. Prove me wrong if it doesn't.