Let's see:

Rome's Hastati-Principes-Triarii legion didn't work particularly well against Hannibal. It didn't work well against the Gallic tribes. It failed against the phalanx at Cynoscephalae. It was terrible in terms of manpower conservation.

But it was flexible, mobile and good if the fighting came to swords. The pilum was very good at pinning enemy shields together and rendering them unusable.

If Rome had expanded to China, the manpower would've been enormous, and the weight, ferocity and skill of the Romans would've had the Han begging the Romans for mercy.

Which, of course, the Romans would show none. *insert evil laugh here*