I had given up on New Frontier a few books ago because Peter David's writing just wasn't working for me. I couldn't pin it down to any one particular thing, just that I wasn't enjoying it as much as I used to. When I read the reviews of Before Dishonor I felt relieved that it wasn't just me.
It's not just you.
I still read and enjoy New Frontier, but he is nowhere near as good as he used to be. Cartooney, over-the-top characterizations and situations seem to be the norm, and the humor is less subtle and more corny / ridiculous than ever before.
I used to particularly love his characterizations of the TNG crew. They are kind of a dry bunch, yet he livened them up (while still nailing the characterizations).
That is also probably why I disliked BD. The crew dynamic is so different now and all the characters just seemed so off, especially the new, barely established ones (obviously). He took them in a very different (and very lame) direction compared to the prior novels.
I was so excited to hear he was returning to the TNG universe, and in a Borg novel no less. If I had known how much of a train wreck it would be, I'd have thought twice.
It is possible too (probably likely) that the bar is higher in Trek lit today than it was in Peter David's heyday in the 90s. There are so many other fantastic writers now its just harder to stand out.