OK, so there may be a place for story-critical on a PL/AC (as long as you're not playing a game in which cheating death is also out, e.g. a survival-in-gritty-circumstances game). And maybe it should carry some small cost, or maybe not.
Let's go back to the drawing board. In building a PL/AC, how much have you found balance to be a concern with the options you choose? Do you find that how you balance building what they can do mechanically (around what the party already has) compared to what you'd like them to do thematically differs to when you're building a PC? In games that you're running or have run, have you found yourself having to say "no" to a lot of things out of balance concerns, or wished you had? Does it irritate you if what a PL/AC can do is out-of-step with what the PCs in the party can do (in either direction), whether you're in the party or running the game?
That ought to do for now...