those d@mn arrows are driving me insane , but none the less you underestimate the legionaries tough to the bone and their training could be almost as brutal as the war itself (no death, but you get the picture), and on contrare Rome would have it easier too get troops and replacements and the chinese would have a harder time recruiting men, why?? well why was it so stable for such a long time? because they had something to offer to the conquered men, you become a soldier for a certain amount of time and at the end of their service they would be granted Roman citizenship and all its privileges, as for the chinese most of the recruited men from conquered territories where forced, and i doubt they would fight at there fullest assuming they didnt try to abandon. and last but not least remember the quinquereme, i can assume that its a bit easier to replicate crossbows than quinqueremes, i rest my case.