It seems to me that there are three main storylines going on here, and two of them work for me and the third really doesn't. 

The two that work both really lean into exploratory elements. In the first, a human/Federation captain clashes with his alien first officer. The alien, along with a handful of his compatriots, are assigned to a shared ship not very long after first contact between the two civilisations. They both have extremely different ideas of what constitutes acceptable behaviour and Carey makes a genuine effort to present both sides as decent, rational people who dislike the ongoing cultural conflict but can't avoid the necessity of it. It's more tension-filled than it sounds, but the challenges of truly engaging with an alien practice and way of thought is fairly well-done. Also well-done is the third civilisation, who - after being stranded in an environmentally impoverished dimension - have built a society around survival and sacrifice. It's not an always pleasant society, but it is, under the circumstances, a convincing one.

The problem is when these two storylines come together. There's really nothing about that particular part of the book that garners any interest. Compared to those two initial strands, it lacks a sense of emotional or narrative credibility - at least it does for me. I can't help but think that there might have been two excellent short books here, if only they were allowed to stand on their own instead of being mashed together and forced into a less satisfactory whole.