The fact that the Son'a are such open-and-shut baddies from the very first time we see them.
You know, good movie villains trade on ambiguity. Heck, even the Borg Queen had a more justifiable reason for her actions than the Son'a. Even Khan had that little thing about Marla McGivers' death, that gave him pathos and something we can empathize with. What have the Son'a got? Nothing. They're narcissists whose plain motivation is their own well-being. They're drawn as such BAD baddies that it stretches credibility that Starfleet would ever be stupid enough to fall in with them, Dominion War or no Dominion War.
I mean, even if the movie had actually played the old predictable switcharoo and played the Son'a as being sympathetic to start with but only gradually unravel and become more villanous as their real plans become obvious, that I could've accepted. Or if Admiral Doughety had been portrayed as being duped along with the rest of us. But the movie plays the Son'a as villains from the very moment they first appear on screen, and Doughety is likewise painted as a corrupt Admiral. Sure, it's broad strokes. It makes it easier for dumbasses in the audience to understand who the good guys and bad guys are in this movie because they're hitting you over the head with a sign reading "THIS GUY = BAD" all the way through. But good characterization it was not. Good writing
it was not.
When even Riker and Troi's research into the Son'a leads them to wonder aloud why we're in bed with these guys, you know your story has gone wrong somewhere along the way.