I think I like what Gege tried to do with Mahito, a fearsome enemy, a perfect ying for itadori's yang. The problem is that until Shibuya, he never felt particularly threatening.
I think he should have had Mihito kill (or wound) the nail girl and fight more.. Intensely before Shibuya. Instead the fight with Nanami he felt strong but still couldn't kill Nanami, it could have been nice to have him show a more ruthless character there.
Instead in Shibuya he does everything back2back, making it a bit rushed to generate fear for him so quickly.
I think he should have had Mihito kill (or wound) the nail girl and fight more.. Intensely before Shibuya. Instead the fight with Nanami he felt strong but still couldn't kill Nanami, it could have been nice to have him show a more ruthless character there.
Instead in Shibuya he does everything back2back, making it a bit rushed to generate fear for him so quickly.
As Itadori blooms from arc to arc, power-wise, so does Mahito. It's just that Itadori is the MC so his villain counterpart needs to develop at a bigger rate to feel threatening. That's why Mahito has a Domain, a very well developed technique, and even a transformation.
Meanwhile Yuji has CE control, Divergent Fist, and is the most blessed character when talking about Black Flash usage.
but your general point stands - Gege doesn't like delving deeper into his world and strictly sticks to his plan.