Personally I don't find the quality poor, but they are very skinny and therefore I'd imagine fairly fragile if they are 'bent' into place while fitting, or the rear seat tube mount is overtightened, which could crack the guard at this point.
I've had mine off and on several times without a problem, but can suggest some fitting tips to help - I didn't work this out by myself, the shop did it and showed me how and why!
1. Unbolt brake calipers and let them hang by the cables.
2. Suspend bike (or at least one end) off the ground and remove the wheel with the bike right way up.
3. Attach the guard by reattaching the caliper throught the mounting and try to keep the guard straight - which is why hanging the bike the right way up helps.
4. At the rear, attach the guard to the seat tube mount, gently tighten the screw - the rubber grommit means over tightening is easy - don't do it.
5. Refit wheel into the frame\forks and guards and take the bike down.
6. Loosen the tops of the guard stays and attach the stays to forks\frame - keep them loose at the top of the stays.
7. Gently adjust the length of stays to get even clearance over the tyre and tighten them up.
One of the most annoying bits is when you find the guard is not aligned with the tyre. Loosening the caliper bolts and re-tightening while trying to keep the guard and caliper central is a pain...patience is a virtue here!