Warning: contains spoilers for My Hero Academia chapter 347 and My Hero Academia Vigilantes chapter 121!
Everyone's favorite teacher from has a top-tier quirk that lets him disable any opponent's powers just by looking at them. While he's shown that he's quite aware of the power's weaknesses, like the need to keep his eyes open, or maintaining a line of sight on the target, there's one more weakness which Aizawa seems powerless to overcome.
Despite its status as a prequel, Aizawa has been a fairly major character in My Hero Academia Vigilantes, acting as a bit of a mentor to that series' protagonist Koichi in addition to performing the usual work expected of a pro hero - at least, what was expected of him before he became a teacher. Vigilantes reveals that Aizawa's trademark goggles, which he uses to protect his eyes and hide his line of sight, originally came from his childhood friend Shirakumo, and marked a major turning point in how he could use his quirk. Given Shirakumo's ultimate fate, the goggles take on a more personal meaning, but in both respects have been a key element of how he sees himself and his role as a hero.
Once Aizawa arrives to lend Koichi a hand in the battle against the out-of-control Number Six, it seems like the fight should be over. After all, if Aizawa looks at Six, that should deactivate its giant flame-body quirk and leave the villain a burned-out mess. Instead, Aizawa grabs Koichi and flees momentarily, explaining that there's one more weakness to Erasure: a sufficiently bright object becomes nearly impossible to lock his gaze on, even if he can keep his eyes from closing due to the brightness. Apparently, being able to make out the distinct body of the quirk is an important aspect of how Erasure works, and Six's nova form is simply too blindingly bright for Aizawa to do his thing.
While this weakness is both logical and interesting on its own in the context of Vigilantes' story, this reveal may bear some relevance to the final fight that's simultaneously occurring in the primary My Hero Academia manga. There, Aizawa's ability to use his quirk has been severely reduced due to various injuries received in past fights, and as a workaround, class 1-B student Monoma has been utilizing his copy quirk to keep Erasure locked on to Shigaraki. Aizawa and Monoma clearly spent some time training to ensure that the student understands the limitations of the ability, but Aizawa's weakness to bright light hasn't really come up in the main series.
While Shigaraki is being aggressively kept from utilizing any of his quirks, the most recent chapter of My Hero Academia showed that Erasure doesn't do anything to unusual abilities that aren't biologically "Quirks," like the mass of fingers he grew. If Shigaraki is able to generate some kind of bright light via biology or technology, or even fly in front of the sun, it could cause Monoma's copy of Erasure to fizzle out and leave the deadly villain unrestrained.
The mass of flesh that spawned off Shigaraki's body has already been pointed out as being particularly deadly, since his Decay quirk is activated through his "hands," and these mutant fingers might technically qualify. Even a moment of failure at erasing his quirk could cause instant death for every hero in the battle. It's a lot of pressure to put on young Monoma, and with this new weakness of Aizawa's Erasure exposed, it places the My Hero Academia team's plan on very thin ice.
