I believe the inherent flaw to these, is that they squeeze the rail on a limited area. That lessons the surface area of contact and also causes stress on the screw itself. The older ones you describe, have stripped out when abused or overtightened.
When you live in harsh climate change areas, you need the best to hold up outside. I imagine it's different in certain parts of the country. We go thru 100 degree temperature swings up here. Add to that how hot the rail gets above the air temp??, directly in the summer sun.