"Nerf" my ass why tf would Oden/Zoro use a weapon that nerfs them against Kaido?
Christ are you even listening to yourself? It's a superior weapon with harsher user conditions. This is not hard to understand.
Enma without it turning black is already a superior weapon than Ryuma's black blade Shisui.
Enma's ability to extract insane amounts of haki is the reason why it's so good. No other known sword can do this.
There is a reason why the weapon grade system exists, why when Zoro got Shisui his attacks became far stronger against Oars and when he got Enma Kaido and BM were freaking out over the sword.
If Zoro fuses with Enma for whatever reason then it will be because its Zoro and Enma not Zoro and Shisui.