The Chinese manufacturing mentality is "make it as good as needed to get the job done and no better." Since WWII, when they started building up their modern military, all of their Russian cloned vehicles and weapons worked, but not as well as the Russian manufactured versions. There is a marked difference in quality, fit and finish. It's not an entirely illogical strategy - more arms minimally meeting service requirements are more valuable than less arms exceeding them.
That said, Nanchang Yak copies seem to be fairly solid and you see a lot flying here in the US. They do seem to put a little more effort in to their military aircraft.