I have been using the plastic ones as tarp tie downs for over 2 years now. I camp on average of about 45 days a year, so figure about 85-90 days of uv/outdoor exposure. Never had one break. But the real stress on the ties downs is not particularly significant. I used the small aluminum ones on the ridge (4 grams each).
My only complaint is that, though they grip fine on 2.2mm Zing-it, they are too large to hold teh 1.75mm Zing-it. And of course, I just switched from 2.2mm to 1.75mm. I now have a set of Dutch Tarp Flyz on order, which are made for 1.75mm line.