As I have followed this debate, only two arguments I have seen have been altogether without foundation.

Superior Japanese generalship is untrue. As with all armies in all time periods, there were great commanders and miserable ones. Most were somewhere in between. Overlooking the obvious Roman giants is stupifying.

Metalurgy would not be a factor in any combat between these forces. How exactly could it be? The gladius and the katana were BOTH just swords albeit requiring radically different techniques to employ. I can't understand why anyone would believe that the grade of steel would make one bit of difference as it plunges into one's body. And no, a gladius would not shatter if stuck by a katana any more than a scutum would.

The Romans did not use pure iron weapons because they could not make them as such even had they so desired. Carbon inadvertantly gets into the iron during the smithing process resulting in a grade of steel quite good enough to serve as a potent sword for any age at all. Steel quality is important for firearms and especially artillery. It can also make a weapon lighter and hence handier. But if battering in armor is the goal lighter isn't so good. Mass is the answer then.

And as a final note, I am unwilling to concede that a legionary could seldom best a samurai in single combat. The defeat of Gaius would be far from certain. Samurai were not gods. Roman troops could defeat Gauls and Germans in single combat despite formation fighting being the norm. Single combat was not unknown to them. They even had awards for it.

A battle between Romans and samurai would be a huge melee. And anything can happen in a fight like that where things get so wild and crazy.