I really enjoyed this - both the Venetans and the Tzenkethi were interesting, and I do enjoy the more overly political books. If you don't like the political side of Treklit you're probably not going to enjoy this. This and The Never-Ending Sacrifice are probably my two favorite McCormack books, and I'd look forward to any of her future outings.
I understand why Elfheny would find the Tzenkethi civilization appealing, as awful as I think it would be to live in that civilization (at least at the E or EE levels that we see.) I think that section of the story worked because it was a Cardassian spy and not a human. I could understand why Elfheny wanted to stay - her own civilization is devastated, and Tzenketh is comparatively resource-rich. As other posters have pointed out, the totalitarian Tzenkethi system is something that Cardassians could only aspire to, so it makes sense that some Cardassians like Elfheny would see it as the fulfillment of an ideal instead of a totalitarian nightmare. Elfheny herself seems happy with her life as an E-grade worker - she has the internal monologue where she thinks about losing herself in the work - and I understood her choice to stay, especially when given the choice between giving herself over to a system she admires versus turning a friend over to that same fate.
If anything is frustrating, it's the intentional ambiguity of things but that's rather the point. How unbalanced is Alden? Is he going to find the extra meaning he needs in helping Cory? Who planted the bomb? But not getting definitive answers to those things is rather the point of the book; everybody ends up unsatisfied. Even Elfheny's early speculations - are all Tzenkethi unhappy underneath a mask of contentment? - stays unanswered, as the only Tzenkethi we really meet are Cory and the enforcers. Of those, Cory is obviously not entirely satisfied, and Artamer seems prone to question things but not terribly discontent.
I do have to wonder if Alex and Neta are relatively incompetent spies, given Cory's comments about them and the enforcers' confirmation that Alex was under suspicion the entire time he was there. If they're representative of the typical operatives in Tzenkethi space, I have to wonder how they haven't all been discovered yet. There's even speculation - I believe by Alden - that Alex wasn't in that great a danger of discovery when we know from the enforcers that he was already under suspicion and had been warned about reconditioning once already. (I also have to wonder how you insert spies into such a closed society in the first place.)
I'd like to see more of the Aventine crew, and hope we get an eventual book or two that actually features different crewmembers and more character development for them. I enjoyed seeing Ezri as captain, but I'd love to see more of the Aventine as a whole.
I did enjoy the look at the Tzenkethi, and would love to see an illustration. I always like having some sort of visual reference for alien species. I think a comparison to the showcase on the Breen in Zero Sum Game is more apt than to the Cardassians in Never-Ending Sacrifice: this gives us a closer look at a Typhon Pact member through a lens of espionage.
This is really minor, but I could've lived without the updates at the beginnings of later chapters about the Venetans' preparations for conflict - the warnings about fear, need for water collection, etc. I didn't feel it provided much of an extra insight to the Venetans, and the text did a better job of building suspense and a possible lead-up to conflict than those little pre-chapter summaries.