Metallurgy. I have never heard of a first hand or published report of a receiver failing metallurgically after an upgrade has been performed and non-defective factory ammunition was used. Chinese receivers surprisingly enough, are forged, not casted. This is a more expensive and labour consuming technique. I find this to be a plus, seeing the fact that the original M14 receivers were forged also. The Springfield and Armscorp receivers are milled from castings.
In regards to the usage of non-factory ammunition, it has been made known to me by Gary Jeter of the VA State Rifle team that,
"We recently cracked several Springfield M14 receivers. Heavy loads with 190 grain bullets were suspect, as the receiver cracked where the bolt strikes the receiver at its most rearward movement."
So, if you decide to use hand-loads there is a possibility of a metallurgic failure with the Polytech/Norinco receiver. But then again, anything is possible. It's just that using factory ammunition allows for liability (can we say "LAWYERS," kids?).