If you read the texts I linked to, that is one of their main points. It is very very hard to compare the katana to a longsword or rapire. There is simply not a standard katana, just like there is not a standard car.
Both the katana and longsword are very effective weapons within their own culture.
But they both have strengths and weaknesses.
And they where developed within their “world”. to deal with other types of weapons and armour within their own world.
(Europe did have more war against other cultures and over a much larger area then Japan did… but it is still a martial culture that is focused on it self.)
But it important to understand that a sword (katana or longsword) can’t cut true plate armour.
The royal armory museum in Copenhagen (Denmark) have two full Japanese plate armors from around 1600.
(and the archaeological museum in Cambridge, UK got two in the same style)
Iam sure they provide very effective protection against the expected (cutting) attacks from a katana.
But they have huge areas with no metal protection. inside of the arms and armpits got no metal at all. Also the face is less protected that a European armor from the same period. But its areas that is very hard to hit with cuts… but stabbing attacks with a longsword would be much very effective. And much more effective than against European armor.
but since a samurai would not face that kind of weapon, it is not relevant to wear an armour that protected against it.