I'll agree there are some issues with act 3 having noticeably less reactivity to stuff like your character's class and race than previous acts, as well as it being kind of huge and easy to get lost in, but I think people do overstate how bad it is in a lot of ways. There's also the practical matter of how much reactivity you actually want before it gets annoying- it seems likely a lot of Flaming Fist would be quite suspicious of drow in the city, for instance, but having them harass you every 10 minutes would get old fast. It'd still be nice to have some extra options in dialog here and there, though.
But I genuinely don't get the complaint that sidequests seem strange given the urgency of the situation- not because it isn't true, but because it's true of the entire game, not just Act 3. it's just the nature of games with sidequests and somewhat non-linear stories.
E.g. Stopping to help the refugees in the druid grove when you have a mind flayer tadpole in your head that, as far as you know, could transform you any day now, doesn't make any more or less sense than any of the sidequests in act 3 with its equally urgent problems. It's just the nature of RPG storytelling and sidequests.
So regarding your point about urgency, I think the difference when it comes to act 3 is that it's getting to the end of the story. It's where we have all the information and things should, structurally, feel like they're coming to a head. Instead it's the part of the game where everything is the most disconnected. Also, unlike in act 1 we have a very clear understanding of how to achieve our goal from pretty early on. We know thatwe have to kill Orin and Gortash and take their stones. Those are simple goals that feel straight forward to achieve compared to the vague "find a healer who can remove the tadpoles" goal of act 1. So I think act 3 is worse due to scale and position in the narrative.
