Had an incident recently that speaks for the durability of those Planet Bike plastic fenders.
I got rear-ended by a fellow cyclist who got momentarily distracted and didn't see me come to a stop. She plowed into the back of me, hit my rear tire, and pannier, and fell over. Poor kid.
She was not seriously hurt, but my rear fender, as well as one pannier got knocked askew. I thought there was damage. Fortunately nothing was broken on human or bike. The fender had accordioned up under my rear rack, and after unjamming it, the fender popped back into shape again. One of the fender stays got bent a bit, and one came off completely, but a ziptie secured it again.
There's only a couple of small creases on the fender where it folded up into itself. Amazing, I thought for sure the fender would be a goner.