I used the nylon Zip Ties on my fenders too. Although the rear metal fender clip is a bit sharp, i can get about 4000 miles or so before I need to put on a new ZIP tie there. Probably if I used a bigger Zip tie I wouldn't have the metal clip slowly eat it's way through.
But what the heck Zip ties are cheap, last a long time, easy to use, and don't rust either.