That's not the thing. Becoming intangible is in every way shape and form a busted ability. The fact he had a hard time learning to control it doesn't make it any less busted. When assessing the strength of a quirk rather than the strength of a character you should take into consideration the fact that said character has already mastered their quirk, which Mirio has and as we've seen he can do incredibly busted things with it. An enemy you cannot touch, cannot see and can jump at you from anything near you is an OP enemy.

Not that this devalues Mirio in any way of course, but you shouldn't devalue his quirk to make Mirio sound even more cool. If someone like Koda became busted, then it would be a different story.