Aside from the Steel Watch Factory, and Gondian Prisoners I enjoyed Act III. Lots of cool stuff and it was fun to complete the companion quests. I agree with the issues around pacing but feel the developers were caught between a choice of; A) more logical pacing and less Act III content or B) more content and illogical pacing. I'm glad they made Act III as large as it is.
As far as the Emperor ...meh it's fine. Larian isn't the first storyteller to have trouble creating a memorable ending.
when i was 18 years old, i will say act 3 is good, just i find emperor is a squid that makes me feel not good.
when i was 28 years old, i will say act 3 is good but i feel sense of dissonance.
when i am 48+, i say emperor put in a very wrong place, since the chess game maker aims to know "the value of mortals" via tav.
the chess game maker won't maneuver your trump card via a playing tricks creature.
i would like to see the myth --
an illithid myth of an entity called the Adversary, an illithid whose partial personality was strong enough to subsume the mind flayer's personality and sought to overthrow illithidkind under the innocent guise of a fellow mind flayer.
the myth implements on tav. and with nether spell, tav even can keep his/her original body if he try to against his/her tadpole.
this is, i think tav should be able to control his trump card -- the prism at least gaining the control in early stage.
whereas the original dream lover -- daicy who is tav's tadpole, this is a much reasonable and better setting.