Why are NPC parties so bad?

[Daralii]

In my experience: Squadron members lack individual personalities and are not scripted to do mechanics, hence why they sometimes seem to take no damage from failing them. Trust members do have individual personalities and are scripted to do mechanics, but they tend to follow a pre-set plan to which you must adapt -- and occasionally, that plan includes standing in the bad (it'd be awfully boring if they never, ever made a mistake).

I vaguely remember something about how you were supposed to follow Y'Shtola's lead against Hydaelyn because she was set up to always do the mechanics correctly.