I really quite enjoyed the texture of the world building of the underhive and how grounded it was in a world of survival and desperation, a cityscape gone completely wild, not overrun by nature so much as man's own savage nature.
I enjoyed the majority of the characters as well, save sadly for Tempes Sol, who seems to be the primary protagonist of the story. I found his character to be almost entirely one-note for the vast majority of the book right up until the end, where I felt it was dropped on the reader as a surprise that "oh look, he actually did develop, he's just kept it secret really hard." What sours me about that is that I liked the direction his character went there, I just wish we got to enjoy the process of it more and that version of his character.
For everything else, i.e. the other characters, the environs, and the interaction between, I enjoyed it all thoroughly.