yeah, i really love powergrips, but i will readily admit that the hardware leaves something to be desired. my general impression is that, more than weight, shoe size will determine how quickly you'll slaughter them. the more your foot hangs over the side of the pedal, the more stress you'll put on that protruding metal piece.
another thing to mention which i've noticed a lot since i've done more clipless riding, is that pedal strike happens sooner with the powergrips than traditional retention systems because of that same metal hardware.
definitely check frequently to make sure that the stramp clamp is tight periodically and you'll be fine.