My Hero Academia's Mirio Togata -- also known as the hero Lemillion -- is only in his third year at U.A. High School, but already he seems destined to become one of Japan's best pro-heroes. While he may have lost his Quirk to one of Overhaul's Quirk-erasing bullets, it's almost a given that one day he will be able to return to heroism work thanks to Nighteye's dying vision of him becoming a great hero. When that day arrives, he'll certainly be a force to contend with given his formidable Permeation Quirk and the skills he gained under Nighteye's stern tutelage.
Fans have been deeply impressed by the tenacity Mirio displayed in Season 4, especially his feat of surviving against Overhaul for five minutes while Quirkless. His Permeation Quirk gives him near invulnerability against his opponents while it's in effect, understandably leaving many to conclude that Mirio is one of the most powerful heroes in the series. But as much as Mirio's skills are hyped up, his weaknesses are often forgotten -- weaknesses which leave him no match for many villains that have appeared throughout the series. So how powerful is Mirio, really?
As mentioned in Season 4 of My Hero Academia, Mirio didn't always stand tall among of U.A.'s Big Three. At first, he lacked the high level of finesse necessary to bring out his Quirk's vast potential. Permeation requires significant skill to use because whenever Mirio becomes impermeable, he loses his sensory capabilities. That means that while Permeation is in effect, he cannot see his foes, hear what's going on or use his lungs to draw breath. The latter is particularly important because it means Mirio cannot remain impermeable indefinitely. It was through his predictive training with Nighteye that Mirio learned to compensate for these drawbacks. He's a fast hero with some of the best control of his Quirk that we've seen in the series. That much is undeniable.
Mirio would do well in match-ups against many standard villains in the series; it's against villains with any amount of defensive strength that he starts to struggle. If he can't get the drop on a villain, his ordinary human strength means he's limited in what he can do against them if they aren't fazed by his punches. It's true the villains would have trouble striking at him in return, but all they would need is one lucky blow or the endurance to outlast Mirio's stamina to take him out.
This means Mirio would lose against villains like Muscular in Season 3 or O'Clock in the Vigilantes spinoff manga. As determined and courageous as Mirio may be, there's just nothing he could do against the overwhelming power of these villains; no blow he'd land would give them a moment's pause. Another common enemy in the series, the nomu engineered by Dr. Ujiko, tend to have defensive or regenerative capabilities. Depending on the Quirks of a common nomu, there's a still chance Mirio could eke out a win, but it would be a tough fight. The high-end nomu such as Hood would absolutely wreck him with their advanced intelligence, raw power and arsenal of regenerative Quirks.
Finally, there's another set of villains Mirio could never dream of defeating on his own -- All For One, Tomura Shigaraki and Gigantomachia. These three currently stand as the most formidable in the series. While this may be obvious to some, it's worth pointing out that each possesses an array of powers that have given even the most powerful heroes of Japan -- All Might and Endeavor -- trouble, and would certainly obliterate Mirio if he tried to take them on alone. Gigantomachia's sheer size and capacity to wreck entire cities in minutes would render Mirio helpless to stop him; Shigaraki's ability to create Decay waves would instantly kill Mirio as soon as he tried to phase in or out of the ground, and that's not even mentioning the other Quirks at his disposal.
It's undeniable that Mirio has significant weaknesses as a hero. There are so many villains out there who would prove to be a bad match-up for him that even his Plus Ultra! level of determination wouldn't make an ounce of difference. However, he's not the only hero in this situation, and that's part of what makes My Hero Academia so engaging -- the complex rock-paper-scissors nature of its hundreds of Quirk match-ups mean that almost no hero can do it all.