In the end, both BM and Kaido actually only got 1 arc each.
We had 0 development for them outside WCI and Wano.
Dropping Kaido's name in PH and waiting until 140+ chapters into Wano itself to give him a half baked flashback is most definitely not "build-up".
Don't get me wrong. I don't like it. I think Kaido could easily have been a "MCU Thanos". But I just can't read this arc and say in my right mind that Kaido was well developed by Oda. Kaido's plan sucks ass. It's stereotypical cartoonish one dimensional villain tier.
BM had a lot of potential, but Oda decided to clown her at almost every change he got. She's not called "Big Meme" for nothing. And people still wanted her for a third arc as if Oda would suddenly start treating her with the hype a villain of her caliber deserves.
It's the same denial as people rooting for Kaido to suddenly get up and get some new awakening mode just to drag this shit show even more.
I'm gonna say it again.
Kaido and Big Mom are the filler Yonkos.
Shanks and BB might not get 140+ chapters arc each, since Oda REALLY wanted to draw One Piece's Japan, but, narratively, comparing these two with Kaido and BM is just not a thing.