I am dissapointed about kaido's design not his personality. When i first saw his silouette by garp, i had in mind a mysterious, charismatic character with unique design. Instead of that, we got the typical hercules-tulkas almighty barbarian. I dont consider him a bad character at all, but i dont think he is great either.
Big mom is okay, i dont find her unstable personality as bad as many fans think it is.
Both of them act as final bosses-type of villains. They run empires and to do that you have to act as an emperor. Thus the mafia theme they both seem to embrace. Their militaristic nature makes their character more "restricted" in comparison to the shichibukai.
Thats the reason people like more shanks and bb, bc they are unpredictable.