I haven't had a single problem with the stand. It's stable and holds well. Mewtwo is in a flying pose, and has survived Mabel knocking around, while Greedo—who stands on his own—is constantly knocked over.
Ringman got the Monsterarts Xenomorph and it has the same type of c-clip stand. When he saw and experimented with the balance of Mewtwo's stand, he was impressed. His Xenomorph isn't displayed, but he could pose it similarly with it essentially "levitating" via the stand.
I'd like to get more c-clip stands as they'd be perfect for the 6" Black Series SW figures. Greedo wouldn't topple so much if he had some balance.
I'm actually beginning to wonder if you happened to have just gotten loose c-clips. Have you tried replacing them with tighter-fitting c-clips?