Quick-take: Complicated. Confusing.
The book starts with Captain Kel Cheris shutting down a mini rebellion. For her success, she is asked for advice to stopping a bigger problem: Heretics have taken over the Fortess of Scattered Needles. She offers up something dangerous and unconventional: She suggests bringing back the genius undead traitor General Shuos Jedao. The council accepts this request and, in a weird unique way, makes Cheris the caretaker of Jedao.
I like to think I am not afraid of complicated books. After all, I read and enjoyed (at least books 1 and 2) the The Collapsing Empire. Ninefox Gambit... I just don't know. I felt like I was on the cusp of understanding what is happening. That is a normal feeling for me when diving into new Sci-Fi or Fantasy. That feeling stays for a couple chapters, and then something clicks and I am to enjoy the story now that I understand the lore. That never happened with Ninefox. I only almost understood throughout the entire novel.
A universe of cultures and technology was painted: exotic weapons, structured classes, programming, ruthless tacticians, differing calendars, math geniuses, large mechanized moths, etc. Something that about the "villain" culture that amused me... You know you are serious about being anti-establishment when you actually name your society "Heresy".
I think this book was painted a bit too detailed. Once I thought I understood something, a new term was thrown at me. I'd just have to mentally set into a category (some gadget, some weapon, or something culture-related). Then I'd move on hoping I could catch on later.
The majority of the book was about Cheris / Jedao's efforts to retake the Fortress. Cheris is thrust into high level command with little preparation and Jedao is helping in his own weird way. I think overall the book is interesting and entertaining. I don't need the sequel. I don't feel like I fully understood the first entry.
Score 3/5. Scale back the sci-fi a little. Use a few normal terms ideas. I think the premise and story would then be easier to follow and more enjoyable.