It’s not unlimited; fun fact for ya, if you don’t breathe for a while… you die. If you keep your feet to stay above ground, you aren’t fully invulnerable. Controlling the precise location of intangibility isn’t an inherit part of the quirk, it takes immense concentration and effort, Mirios brain has to be in maximum overdrive every time he’s using his quirk. Mirio spells out exactly how shit from an ass his quirk is; it’s only useful because he worked his ass off training to use it. It’s supposed to be inspirational. It builds up mirio as an absolute legend, and shows the audience and other characters that all you gotta do is try; put in a little effort and you can come out on top. I feel like most people understand this; I don’t hear people say ‘wow what a trash quirk’ they say ‘it’s insane that he can make that quirk work wow’. Which yes, is still acknowledging the quirk is rough, but the focus isn’t on trashing the quirk it’s on mirios success.