The crews. Americans make good sailors, chinese don't.
Chinese might be smart to hire Vietnamese to crew their subs -- that would work better than Chinese.
Good point. I remember on some NATO maneuvers, the shadowing Russians were trying to refuel at sea and were still fumbling after half a dozen tries. Two of our ships came nearby and refueled in one pass (Nanner, Nanner!) to show 'em how a REAL Navy does things.
Then there are all those stories, ranging from armour to aircraft, where the Third Worlders were given/copied high tech equipment and then could neither maintain them nor operate them competently.