When they made my life difficult, when they made me angry, and I couldn't do anything about their behavior, I finally told them how I felt, and let them deal with their own reactions to that. When it was clear no good would come of it, I just stepped away. Stopped going along to their house, stopped being home if they were visiting, stopped calling or emailing. Stopped responding to their complaints and suggestions; stopped relying on their promises. After a while I learned to stop bitching to my XH about them, too -- and if he had problems with them, I stopped listening to his complaints, and told him to find his own way to deal with them, since we were already into multiple reruns and he was after all a middle-aged man.
The truth is that a) I don't know them well, or know the world from their POV; b) they're good to my daughter; c) now that I'm gone, they're willing to help my ex; d) I believe their presence during visitation leaves my daughter better off. That's good enough for me. I have little idea what goes on during their visits. If I see that there's some sort of damage going on, obviously I'll step in. I trust my daughter is bright and honest enough, and close enough to me, to say something if they put me down to her. But it seems to me my dd comes home happy and with presents, and feels loved, so as far as I'm concerned, there's no problem. And no reason to get snide about them.