Unfortunately, I agree that a lot of the game falls apart after Act II.
For me, the adventure and city don't carry the mood forward properly.
--there's a lot going on in the city, but it doesn't come to life because it feels like it has more of the modern writers' sensibilities than anything true to the setting.
--the meeting with Gortash happens too fast. It should be something that's worked toward and that happens in the Upper City after much of the other content has been explored. (Also, Gortash kinda sucks as a major villain. He needs to be touched up, just my opinion.) This would then become the last stop before confronting the elder brain.
--Rivington doesn't project the proper flavor. You don't get the proper sense of it. The squalor, the anger, the resentment, the ticking nature of it all, like the whole place is about to explode. None of that comes across. It tries to at times, but fails. There's a picnic, a happy gnome washing his pits, a squatter who's the "good guy" while the "bad guy" owns the house. It just falls down on itself, the whole area.
--additionally in Rivington, the murder investigation should be *FELT* everywhere. It should be such a huge part of the flavor there. People should be scared, worried, like when a notorious serial killer is on the loose in an urban environment. Getting through the murder investigation should be the best way to get through to the Lower City. As it stands, there are so many ways into the Lower City that it all jumbles together.
--overall, there needs to be a stronger thread dragging you through Act III, and more flavor flavor flavor, making the Act come alive like what we have in Acts I & II.
