In my personal experiences I have had bad luck with a lot of Clearguns.
G&G CAS guns well M4s anyway, the hopup sucks, and lower cracks BAD... gearbox seems good.
KWA M4 - 3 KWAs Ive seen blew the Bevel gear and Tappet Plate.
ICS - Sometimes good sometimes bad, I've had a few bad ones come in broken from factory broken clear lowers and the gear box had issues.
SRC - No Comment never had one, But have heard good things.
The JG from Velocity Arms seems to be having good luck.
I've had many clear JG guns, not VA versions but other companies and they were AWSOME guns, super reliable!
Honest Opinion: Save up and get a new Full Metal from some retailer.
Clear Guns work, but the bodies suck! Break way to easy, and seem to have mag issues with some.