If you’re riding in mud a lot, zips fail.
Haven’t seen out-of-the-ordinary failures with my Endura kit – and I own a fair bit, and it gets a fair use. Plenty of other kit (and kit with YKK zippers) fails on me.
I’ve got a pair of Endura tights which must be about 20 years old now, and still going strong – as are the zips. The tags tend to fatigue and fall off, but they do that on a lot of zips, and I either replace them with new tags or put a bit of light line through the loop on the zip head – come to think of it the vents on my ten year-ish old humvees have line through the pull tags as the rubber heads cracked and fell off.
I had the full length zip on an eight year old thermal jersey fail over Christmas; I mailed them, got a response shortly after the break apologising for the delay, sent in the jersey and got it back yesterday with a brand new zip sewn in really cleanly – you couldn’t see it had been replaced.
Off the top of my head, Musto and Henri Lloyd also offer repair services and have done zips, drysuit seals and stuff for me over the years for bargain prices. Scottish Mountain Gear will also do new zips – they fixed my old Wingnut really nicely when the main zip on that failed.