This is one of my favorite Next Gen books, My favorite Duane book after Spock's World, and My favorite Mirror Universe story. And best of all it's not the least bit incompatible with tv show continuity since there's no reason why there can't be two different mirror universes.
But there are contradictions on the Prime-Universe side too. There's the difference in secrecy that rfmcdpei
mentioned, and there's the problematical claim about a galactic-scale cataclysm resulting if matter from another timeline is permanently transferred across -- which has been definitively contradicted by "In a Mirror, Darkly" if nothing else. (And by Forgotten History
, on a planetary scale.)
But what does it matter? Continuity is nice when you can get it, but it's all just a bunch of made-up stuff anyway. All that matters is if the story is enjoyable.