I think this is the first of Cherryh's 'Alliance-Union Universe' books that I read. I liked it so much, I went out and bought a whole load more of them, and have now read, and own, most of them.
So I'm a bit biased, as I'd probably read a shopping list if Cherryh wrote it. And who knows, I might even end up paying for it, so please don't tell her...
As with some of Cherryh's best books, things do get a little depressing for the main characters, and the reader. Which shows that you actually do care what happens to them.
Not all the concepts are defined as they appear in later books, and this book has a limited viewpoint. But that's the advantage of creating your own Universe, and writing several books in it. Otherwise, every SF book would be as long as 'Lord of the Rings', and we'd have no trees left.
It's got some nice ideas, and some good characters in it. As well as several plot twists that seem natural, and keep you interested. Cherryh does have a tendency to just dump you in a situation and expect you to understand it, so a bit of science, and some SF basics helps, but the story keeps on moving, and I don't think this is too crucial.
The best thing about the book (and Cherry's work in general), is that it doesn't simply boil down to good versus evil, but is a bit more real than that. The characters face unfolding events, and make the choices they have to, as they come up. The book is internally consistent, and if you like it, there's a whole universe of books to explore afterwards.