The success of a mystery rests on its plotting. The murderer has to be someone we met reasonably early in the book, ideally a suspect considered but rejected, and the solution has to be plausible but not especially guessable. For me, outlandish mechanics (locked room mysteries and the like) don’t work in books, although they can on TV, because they reveal the hard work of the author and reading them feels like doing the kind of puzzle I do not enjoy. Watching TV is passive so as long as the pace is decent you can just get swept along.
Exiles was definitely a good mystery judging by these rules, and I also appreciated the lack of jeopardy for the detective. In fact, the detective’s own story was more of a charming, easy-going romance.