I wouldn't be so fast to write it off in Roman's favor, even without samurai army using guns and cannons. I do not recall Hannibal's army of mercenaries having the reputation to excel in advanced strategies, but we still had the bloody outcome at Cannae.

One aspect nobody thought of - imagine the charge of samurai warriors; see them before your inner eye, storming down in their glittering armor, with demon masks on their heads, swinging katanas that could cut even through the Roman shields. Fearless, screaming enraged, charging in the middle of the enemy. The bravery of Roman soldiers aside, I think they would be sure the gates of Hades themselves opened to unleash that demon army.