The Japanese had no weapons advantage over the Romans. All of the melee weapons available to both were more than adequate to kill one another. Differences in melee weapons were almost NEVER very important throughout history. Put a modern soldier with a bayonet on his assault rifle against a man with a baseball bat and the outcome is not a foregone conclusion. Especially if the man has used the bat in combat before and is fearless!

Katanas could not cut through a scutum. No way. Samurai were no braver than Celts and Germans who were also skilled and motivated .

Marian legionaries were pros and formations would give them a huge advantage. Fighting single combat warriors like samurai or pikemen like Sengoku era ashigaru is what the Roman army did for a living.