Its 2 major things imo:
- Not addressing his Oni lineage and how that factors into his whole mindset as an antagonist. We only got an allusion to this with him and Yamato, but theres clearly history there that Oda did not elaborate on
- No "visible" awakening. He talked and gloated about it, yet Oda never explicitly showed it. If he was already using it, why not say it?
Thats really it. He was fine otherwise. Ive said it many times already, but Wano ironically needed 20 more chapters. More Awakening fights in general, more backstory for the villain, more closure for Wano's characters.
That said, he can still technically address this on a future plotline if Kaido isnt dead, but it just makes Wano look worse than an inexplicable with certain things.