For one it runs into paradox of tolerance problems, for another it fallaciously relies on a "marketplace of ideas" to resolve friction which, despite the bumper sticker term, is not a real mechanism.

It's been a longstanding part of the fascist playbook to turn the norms of liberalism against itself, advocating for "free speech" when it helps actively amplify their message to audiences, and having no hesitation to abandon those purported principles once in power and able to censor opponents. Poof, there goes your free speech.

Principle agnostic approaches to freedom of expression lead to the collapse of democracies. Happened in Hungary, almost happened in Poland, and it's unfolding in the U.S. The point isn't that these idea's "win" in a marketplace of ideas but that they mobilize violent anti-democratic capacity.