Questions & Mysteries Why does Oda only draw CoA Hardening for a few characters post-WCI?

[Cinera]

The black hardening has been overpowered by invisible hardening in the manga multiple times. It’s not a thing, it’s always just been a Stylistic choice.

No, because a stronger character's invisible COA can be superior to a weaker character's Hardening.

E.g. consider how Luffy's Haoshoku coating kick didn't overpower Borsalino's invisible COA.