Katana and WHICH european sword? You're trying to compare a specific style of weapon and the entire production of a continent! Differences between a Katana and an old school viking-to-"knightly" one hander, sure. Differences between a Katana and a hefty falchion, alright. Differences between the Katana and a robustly built saber, stretching but I'll grant it. Differences between a Katana and a good Hand-and-Half, though? There are differences in blade shape and particularly hilt design, but by and large they're both agile, versatile two handed swords with pretty good reach for a weapon that can be belt-carried, that are quite deadly in either the cut or the thrust. That's what matters in the context of weapon usage. Are there differences in metalworking techniques? Sometimes, yeah. Europe had as far as I know generally phased out pattern-welding by the relevant time period. But the difference in actual use characteristics from that is limited. A well made sword is durable, flexible, and generally resilient against harm. A terribly made sword is a danger to anyone in the vicinity, friend or foe. Both those kinds of sword exist in both regions, and in both regions the actual warriors wouldn't be caught dead with the latter. European and Japanese sword-smiths both knew what they were doing, as did the people who used these swords. They wouldn't be any happier to go onto a battlefield with a sword-in-name-only than troops today would go out with guns that jammed every few rounds. Given these swords both were in use for centuries, and in periods when the sword was a militarily viable weapon? I have the sneaking suspicion they worked!