Regarding Act 3 I'll have to put it in a spoiler as my last post was edited when i mentioned this.

Larian have a bit of a track record of starting strong and finishing poor, DOS2 and it's final act is a good example, my guess is that the reason for act 3 being so poor is due to the removal of the upper city very late into development, as someone mentioned elsewhere Swen said it was playable 2 weeks before the launch of the game, they didn't provide citation but if the guy comes here I'm sure he can offer a source. The upper city was mentioned as being playable in the community update 19 on steam 2 months before the games release, it also supposedly broke Karlach's quest and ending to which was her good happy ending, the game treats Karlach as if the player ignored her and didn't try to help currently.

We don't know what went on internally that lead to them cutting parts of act 3 so very late into development as Larian have been in radio silence regarding it. The good news is despite act 3 not being good there's a very high chance they will fix it in a definitive edition as they did for DOS2s final act, the downside is it'll take up to a year to see it if we follow their past trends.