Aside from the problem of the scope rotating a bit as you tighten the screws, my main gripe dates back to the days of steel-tube scopes. Once or twice I had oil creep under the rings so the scope could be turned by hand. Probably my fault for using WD40. A minor gripe was the cheap slotted screws.
On the other hand, I used Weavers to mount scopes on 12ga slug guns, and the held just fine. That's a pretty tough test.
