Of all the Trek books I've read, which is the vast majority of them published since 2000, with a few from before that
, the only ones I would describe as "bad" are "Before Dishonor" and the "Rebels" trilogy.
"Before Dishonor" was a misfire to me; it completely undercut the Borg-are-scary-again idea by turning the race in question into a joke (and indeed turning almost every event in the book into a joke), it reduced every character to a caricature without taking into account any of their development across other books or even the later seasons of their TV shows, and T'Lana went from having "remarkable respect for other cultures" in "Resistance" to flat-out "Vulcans are superior to everyone else because our culture promotes emotional suppression" here. The whole thing felt shallow and gimmicky, which was particularly unacceptable given the highly significant event at the end, which required much dignity. Having read quite a few other Peter David books- "Imzadi", "The Captain's Daughter", most of "New Frontier", I know he can do a lot better, so I can't simply say "I don't like the author". I have enjoyed this author elsewhere, even if he's not among my favourites, so this has to be considered a misfire.
"Rebels" was the worst, though. It didn't even feel like Star Trek. The Cardassians and Bajorans were unrecognisable.
There have of course been disappointing or unremarkable books, but I almost always find something to like, so these are the only ones that have left me unsatisfied.

"Before Dishonor" was a misfire to me; it completely undercut the Borg-are-scary-again idea by turning the race in question into a joke (and indeed turning almost every event in the book into a joke), it reduced every character to a caricature without taking into account any of their development across other books or even the later seasons of their TV shows, and T'Lana went from having "remarkable respect for other cultures" in "Resistance" to flat-out "Vulcans are superior to everyone else because our culture promotes emotional suppression" here. The whole thing felt shallow and gimmicky, which was particularly unacceptable given the highly significant event at the end, which required much dignity. Having read quite a few other Peter David books- "Imzadi", "The Captain's Daughter", most of "New Frontier", I know he can do a lot better, so I can't simply say "I don't like the author". I have enjoyed this author elsewhere, even if he's not among my favourites, so this has to be considered a misfire.
"Rebels" was the worst, though. It didn't even feel like Star Trek. The Cardassians and Bajorans were unrecognisable.

There have of course been disappointing or unremarkable books, but I almost always find something to like, so these are the only ones that have left me unsatisfied.