2.5/5, pretty disappointing. I was really looking forward to this sequel, as I thought that the first book was one of the most original and interesting sf books I've read in years, a worthy successor to Iain M. Banks' early Culture novels.
I knew that Lee was up against a challenge after I rereading Ninefox, as the central relationship that made the first book so compelling wasn't going to be present in the same way (I'll be a little vague in this review to avoid spoilers). Rather than facing the issue head on, they simply avoided it, adding new characters that are generally less interesting. This also avoided clarifying what happened to the main character of the first novel, Cheris/Jedao.
If this was meant to build suspense, the strategy failed. What happened to Cheris/Jedao isn't particularly surprising, as it is hinted at by other characters. As another reviewer noted, this renders a large chunk of the novel slow and uninteresting. You just took your top player out of the game!
Worse, the "big" reveal about 3/4 of the way into the novel turns on a major hole in the plot. One of the castes in the novel is compelled to obey superior officers by what's called "formation instinct." Much of the story revolves around individuals who are more beholden to formation instinct, as well as those who lack or can nullify this instinct. In itself, that's interesting. Yet, a character who permanently frees herself of formation instinct is then later showed *multiple times* as once again following formation instinct. It makes absolutely no sense and is one of the worst plot inconsistencies I've seen in a while.
There are some other niggling issues: for being so effective with neologisms, some fall extremely flat. Calling ships "moths" is just silly and gets worse every time. What happened to the grand sf of tradition of creative and evocative names for ship classes (Bird of Prey, White Star, Constellation Class) or individual ships (just about any Culture ship, many from Bablylon 5)? Instead we get cindermoth, lensmoth, boxmoth, moth moth moth. The clunky "manform" and "womanform" could have been handled much more smoothly (Banks does this much better, for example) and ends up feeling more like pushing politics than being organic to the universe (which, while still political, is at least subtle!)
I hope the author can go back, understand what made the first novel great, and carry those lessons forward to future books. My two cents: the trend of ensemble casts in sf and fantasy really needs to die. I get it, Game of Thrones makes bank on HBO, but how many people just flip/skip through to Arya and Tyrion, and occasionally one of the other characters? The Perrin/Faile plotline in Wheel of Time was horrendously and hilariously gendered, stilted, and boring. And the Expanse, while superficially ensemble (the books, not the series), purposefully let only a few characters per novel have voice - even then, you run into stronger/weaker plot lines.
When you compare the A+ plotting and character development of Cheris/Jedao from the first novel to the solid B of Khiruev and Kujen, the C of Mikodez and the D- of Brezan, there's really no contest at all.