Huh. Interesting one.
I think I basically ended up admiring this more than I loved it. It does, however, draw together the threads of the previous two books very effectively and continue what seem to be an over-arching themes of the use and abuse of power.
So I feel this is the book it needed to be and I think it's very ambitious in structure and approach and, honestly, the sort of ideas its trying to explore ... but yea, it doesn't entirely come together as a story for me. Especially because the plot revolves around mysteries, large and small, and Bitterblue (for perfectly understandable in character reasons) isn't all that interested in solving them, for about the first two thirds of the book.
The pacing, in general, felt a bit off to me and there seemed to be long stretches of time with no particular progress or focus - which again, sort of makes sense, since Bitterblue is trying to run her kingdom, not be a protagonist. And some of the sub-plots seemed a bit under-baked: for example Bitterblue's romance with Saf seems pretty shallow (although, something I did appreciate was the fact it wasn't the POINT of the book, nor did it ever become Bitterblue's goal). But there's also lots of the usual Cashore's goodness here: sensible and open-minded attitudes to love and romance (Po and Katsa still aren't married, Bitterblue is allowed to have sex and for the sex to be good, without her ending up in a FOREVERMANCE), lots and varied female characters, all with agency and complexity, same-sex couples depicted with care and respect, and a strong commitment with what you might call a feminist-friendly ideology.
If FIRE is, at its heart, a book about (male) gaze and rape culture: then I saw BITTERBLUE as a book about abuse survival. She is, after all, the queen of wounded city. And the 'happy ending' she and her people eventually forge may not be a traditional one-- it may, indeed, be shot through with loss and suffering---but it is ultimately one of healing and acceptance and truth.