Brook lost his equivalent fight in Thriller Bark horribly. And Law really didn’t do all that well against Doflamingo either.
Jumping the gun here. We know that Yamato will fight Kaido with all she has.
But will Kaido, who is still insisting that he is Yamato’s father and will make her Shogun, fight with all he has against her? Or will he treat her like he did the Scabbards, give her a shot before finishing her quickly without ever needing to go all out.
And is the setup for Yamato to fight alongside Luffy, or to need to be saved by him? If she is to be a Strawhat, it’s more likely she’ll need to be saved.
Plus, even if Yamato goes on to really push Kaido, what’s to say King couldn’t do the same? The upper reaches of both their strengths really hasn’t been touched on yet.
It also has to be said, the fact the Calamities didn’t bother to hunt Yamato themselves and that the task was given to the Flying Six, with the reward then being a chance to fight a Calamity, does not imply the Beast Pirates think Yamato is as strong as a Calamity, at any rate.