@Kaido:
Good authors set up villains who are "invincible" in order for the main characters to find a way to make them not invincible. Anything else doesn't make the story work.
That's exactly the problem with Kaido. A good major villain needs to have this kind of "aura of invincibility". Actually I don't really care anymore whether the scabbards inflict a wound because that aura was destroyed long ago.
To clarify, there are two types of One Piece main villains. Those who are main villains of something bigger than an arc (like Crocodile, Enel, Lucchi) and those who are not (like Mr. 3, Wapol). The latter doesn't that aura, the former however has to have it at all costs. Some cases may be debatable, but we can agree that Kaido is one of these major bosses and therefore needs this aura, right?
However, now comes the big problem with Kaido, that is, the crucial key point narrative-wise is: It's perfectly fine to beat the shit out of that enemy power-level wise, as long as you as author somehow manage to leave that aura intact.
In his intro, Kaido is portrayed as someone who gets beaten and overpowered all the time. On a power level standpoint, Kaido isn't anything that special in his introduction. However, there's still the aspect of him being unkillable, and thus making suicide his hobby, as well as the 1 vs 1 remark - which is crucial as instead of tarnishing the aura, it actually helps increasing it. The "hype through defeat" trope executed masterfully.
Contrast that to the Oden vs. Kaido battle. Kaido losing really is no problem BY ITSELF. However, the scene itself is just poorly written. It completely lacks anything to keep the aura of invincibility intact. Instead we got characters like the old hag whose sole narrative purpose is to blatantly cover up Oda's poor writing. Oda wants to brute-force into us, what an honorable warrior Kaido is by killing the hag offscreen later on, but it just doesn't work at all on a narrative level. Compare that to Luffy vs. Katakuri. Show, don't tell. Unlike Kaido we are actually shown that Katakuri IS a honorable person.
Kaido is piss-weak!
Not because of powerlevel stuff, but because of the poor choice of actions he himself had chosen. Well-written characters are made by their motivation and desire and, that's the important point, the willpower, strength and resolve they are going to muster in order to achieve that goal (or these goals as motivation and desire may vary). That's why Kaido is weak. Because despite claiming to seek a fair and honorable battle, he showed little to no resolve to actually reach that goal in the case of Oden. If his words are supposed to be more than a lip service, why didn't Kaido demand Oden to deliver his second blow instead of suckerpunching him.
Because Kaido is a weak-minded piece of shit, lacking any needed aura of invincibility, which completely does not deserve this major villain role at all.