The real danger of AfO, if we're to believe All Might, is that he's a villain's All Might. By that I don't just mean he's just as strong or potentially stronger, he's just as charismatic. Just like how All Might can capture the hearts of people, it's likely that AfO can do the same. And do that with villains on top of that. Look at how his guidance made Shigaraki that much more of a threat to society. And that's just Shigaraki, who knows how many villains he has trained. Or how many people he has turned into villains, for that matter.
To be honest, AfO would be even more frightening if he had left Japan to steal quirks worldwide. I guess the world lucked out that he went into hiding in Japan
