I don't think I've been more disappointed in anything with the "Star Trek" name on it as I was with The Returned. I was with New Frontier through Missing In Action, but almost every choice since then has been steadily worse and worse. (And I count the first part of NF, from House of Card to Stone & Anvil, as some of my favorite Trek.) Since then, main characters are dropping like flies so quickly that any dramatic resonance is lost, characters are behaving in willfully incompetent ways, deeply troubling moments are treated as if they were jokes... Ugh. Genuinely distasteful.
New Frontier was always turned up to 11, which made writing it a pretty difficult balancing act; it had to be grand and epic and bizarre and hilarious without losing its essential humanity and believability, and I know for some people it failed that balancing act already with the ending of the first four books. For others, they're still with him even through The Returned, not seeing much of a difference between the series's earlier wackiness and its current wackiness. But for me, it has fallen apart completely; it was always a difficult balance, so small changes can make a big difference in suspension of disbelief. Objectively, I don't think his writing has actually changed all that much, but subjectively it was enough to lose me completely.