The safety-release is needed on SKS because the material is not brittle. It can fold on itself without cracking and jam between the tyre and fork.
My QRs have released twice, once after catching a section of barbed wire in my tyre and once with a branch.
The latter accident did fold the fender a bit but I was able to use them for another year (4000miles) before the fold cracked.

The full-length ones are much more sociable than 3/4 length which splatter road muck on riders behind you.
My 10yr old ones feature a plastic mudflap but they dropped it in later versions. You can rivet one made from a detergent bottle material.