[fancifalmanima]

It would depend a lot on what time period you're talking about. I'm not an expert, but I do a bit of amateur forging and have learned a couple of things just reading about this craft. By the 14 or 1500s, spring steel had been developed in Europe. This would enable a sword to flex rather than break. Japanese swords from the time weren't flexible and were generally more prone to breaking. This had a lot to do with the raw materials that were available. They also tend to have softer mild steel core to help with this problem, with a very hard and sharp edge. If the entire blade were made of the same material as the edge, it would be extremely inflexible and brittle. They're kind of designed in a way where a part of the every hard and brittle edge can crack, but maybe it won't extend all the way up through the blade leaving it somewhat usable. The European longsword from the time might just flex in the same circumstance.

That's not to say that European swords were better than Japanese swords in every way. This is one of many, many factors. And I'm sure there were plenty of crappy longswords at the time (and crappy katans), so you kind of also have to decide if you're comparing the best examples, average examples, or low quality items as well. The skill of the wielder is also important. If you're throwing out a bunch of random soldiers without a ton of training and giving them a sword, you might want to give them something they're less likely to break. My understanding that is there was a period in Japan where only samurai were allowed to carry swords (if my reading is to be believed), who were generally very skilled. They would probably know how to avoid putting their blade in situations where it would be prone to breaking.

And Japanese traditional Japanese sword making techniques are extremely impressive and interesting to read about given the materials that were available at the time.