The failures IMO are caused by unequal tightening of the 4 screw design in concert with not enough material thickness.
I meant to mention your point Ben. It's important tightening them to go slowly while evenly tightening all 4 screws on each ring "in concert" as you say. I'd think this sort of stuff is obvious, but perhaps not.
Some people are ham fisted almost beyond belief. In fact a lot of people are.
