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.
Never under estimate the ability of the general public to screw things up!
