Terrible in what way? In terms of what their characters did, how they were played, what stories they were involved in?
Dukat was terrible, he murdered millions.
You could argue that Vorta in general are brainwashed, but we also saw evidence that Weyoun would actually go behind the founders backs (at least setting aside their wishes - as seen in Treachery, Faith and the Great River)
And of course the changelings in general, the Martok one encouraged a massive war, the main "Female Changeling"
In terms of underutilized characters, Jake was probably the worse of the main - it's impressive how well rounded the main characters were while also having as many fleshed out supporting characters, especially compared to Voyager. I think Moogie got more character growth than Tuvok.
Keiko and Molly conflicted with a fair amount of what they wanted to do with O'Brien. I'm glad they didn't just take the easy option and either bump her off or divorce.
I'd generally agree, but he allowed some key episodes to be made - especially His Way and Paper Moon.