I've got 29 unique models of KZ earphones plus dupes of many and have only had a couple failures in the bunch, those happening with old, discontinued models. Not bad for cheapos and better than I've experienced with more established expensive brands. Like everything, they'll last if you treat them well.
Out of ~12(1xED12, 1xZS3, 2xZST, 1x HDS3, 2xATE, 1xED9, 1xATR, 1xZS1, 1xRX) KZ earphones purchased, 1 has been outright defective, 2 have failed since purchase and 4 have been functional but arrived with issues.
Outright Defective -
ED12 - Left side doesnt seal, right one does and sounds different.
Broken postpurchase -
KZ ATE - Jack gave up the ghost after being used daily for about 2 months. Partly my fault .
KZ HDS3 - Connection broke @ earphone entry after bieng used irregularly but slept with. Model has weak connection joints.
Functional but with issues:
KZ ZST - Crackle from one side when creating a seal. Mildly annoying.
KZ ZS1 - Jack was intermittent after first recieving them. Would cut out randomly. This stopped 30 minutes after owning them, don't know what that was about. Infuriating until it stopped,. Gave the pair away after a month of ownership due to liking the ATE much better.
KZ ATR - Drivers arent well matched - It just feels super weird listening to anything on these. Either that or they have better instrument positioning than the ZST and I'm just not realizing it.
KZ ZS3 - Red Earphone grille and some foam fell out the first time i replaced tips. Didnt seem to change sound too much.
So historically their QC hasnt been that great, atleast not for me.
On the other hand, defects have been minor and most are still functional.