It isn't a big issue though - Marco had better regen than Whitebeard, Katakuri had better CoO than Big Mom. It's only fair
Nah it is.
King being more durable than Kaido would be like Katakuri being more durable than Linlin.
King has significantly superior durability to Kaido, far better mobility and maybe even speed.
Kaido doesn't have much of an AP advantage either (his Haoshoku Thunder Bagua couldn't even knock out Post Hakai Zoro).
IMO Base Kaido flat out cannot beat King and even in Hybrid, Kaido wouldn't be putting him down easily (see how easily Law stood up from a Haoshoku Thunder Bagua, and he has neither Ancient Zoan durability not the Lunarian perks).
Given how he's consistently overpowered Zoro, King should be capable of dealing at least Rooftop level damage to Kaido.
Even with just what he's shown so far, King should be able to give Hybrid Kaido a solid medium difficulty fight.
If he shows Advanced Haoshoku of his own, high difficulty enters the realm of reason.