The myth that Katana's were diamond bladed weapons of mass destruction was rampant on the early days of the Internet and has long since been debunked, But man...After all the shit I read about Katanas Idk how much of it is true cause It's getting harder to believe that these things were ever used to kill someone, Now before you say, I know wars are mostly fought using Polearms, And Katanas WERE sidearms, just like arming swords, Im light on history, but I aint sschutpid, Now to the point, Were Katana's and their counterparts really a brittle easily broken pile of trash that was for show more than anything? It's hard to believe anyone could traverse a terrain as jagged as japan with these things, Every article describes them as toothpick swords, And If that was true then why were they used for SO long.
The 'katanas are crap' line is as much hogwash as the 'katanas are unstoppable' one. For further information, we can turn to u/wotan_weevil's multiple excellent posts on the matter:
Japanese swords were plagued by the problem of low quality iron minerals to process into steel. The high amount of impurities forced the repetitive elaboration of the metal before shaping it into a sword, also prompting the improvement of forging techniques to limit the flaws of the metal, like the famous folding of the steel billet.
It is true that they were kept very sharp and continuosly maintained due to their tendency to lose their edge quite easily. We can see a hint of this acknwoledgement of the problem in certain combat style emphasiziing quick and single strikes against a foe with as little contact between the weapons as possinle (most people point at Iaido for example).
Beside the fact that no one in their right mind would have fought a battle with a sword alone (except some "specialized" weapons like the Roman gladius and the no-dachi), no one in their right mind would have attempted to cut through metal armor with a sword. This, added with the problem of low steel quality and therefore the easier loss of blade edge.
However, all swords share this problem. The simple reason is that no swords can cut through metal armor. Even in Europe, if a knight tried to cut another knight's armored arm off his sword would be dented or dulled. This can be seen in some later combat manuals were the Western sword is used in a special grip to strike with its crossguard and pommel as if an improvised hammer, due to its construction.
To sum it up, both the "katana bias" and overjoyed fanboys contributed to the creation of both myths. The truest thing is that the katana metal was of low quality and tended to get dull very quickly, but the very high sharpness granted a very deep and clean cut over the first strikes. There is a similar debate with Western swords as well. Swords were to be employed both as a backup weapon or as personal defence inside a city or palace.