The only area to watch is what Buzzstrike said-the bolts. The metal used wasn't the greatest and they were soft.
I had a PolyTech years ago and loved it. I checked the headspace every 1000rnds or so and had no signs of the headspace growing.
The only issues that I have seen with the Norinco's is that the engraving is really rough and doesn't look that great.
-Don't plan on swapping the bolt for a USGI bolt without modification/work-usually best left to someone who knows what they are doing (Smith Enterprise/Fulton Armory). Even though my headspace didn't grow-if yours does this is a $400+ modification which all told will put you into an M1A, etc.
-The barrels are interchangable between the US and Chinese.
-The Flash Suppressor Castle Nut is Chinese specific (USGI won't work)
-The Gas Cylinder Lock is Chinese specific (USGI won't work)
-Parts within the Gas Cylinder (Gas Plug, Piston) are Chinese specific (USGI won't work). If you want the use the NM gas pistons available, then you'll need to buy a complete USGI Gas Cylinder Assembly.
-The Hammer isn't good for NM mods-something about the engagement points being off or soft. If you want a NM trigger, just buy a complete USGI unit and replace the Chinese one.
The other small parts are swappable between the USGI parts and Chinese.