Swords forged specific way to reinforce them as much as possible because the lack of large stockpiles of metal made them forced to come up with more efficient methods.
The Katanas are supposed to be very hard and retain a sharp edge, but that hardness comes with the price of fragility against direct hits.
They're meant to deflect and redirect blows, not block them. They're meant to be used to hit on weak spots, and samurai armor more often than not had minimal metal components as opposed to western armor.
They break. They're hardly unbreakable, a noob wielding one against a longsword would likely see Katana break and while whining get a faceful of longsword's edge cleaving into their skull.
In the hands of a master? They'd be able to deflect and redirect strikes, but one on one against a longsword, they're -fragile-.
The problem with western swords in turn is that they're meant to strike direct blows and they can dull, yes, but they can be resharpened so just more maintenance on that front.
Samurai Armor & Katana vs a Full Plated warrior with a Longsword?
Katana guy's going to last longer because less weight on them, but longsword blow goes into the samurai armor like it's cheap ass riveted hide armor, katana -breaks- against plate unless the samurai gets lucky and cuts up under the arm or similar strike to the joints.
Going under armor plates, anime exaggeration occurs.
Am I trolling or trying to actually argue? Eh, even I'm not sure anymore. Just saying, both are cool and interesting, but given I get to hear a lot of these discussions from guys who actually practice with real swords I end up laughing at the Katana weeaboo claims all too easily. Ah well.
Still, excellent trollbait to make a guy think for a moment. It would be better for someone that was fanatical enough to rage, but guess we're lacking in the sort of folks here right now.