Example: Anarchist’s attempt to shut down a Milo Yiannopoulos rally at UC Berkeley, look it up).
1. State has the monopoly on violence. If you use violent tactics against the state (doing something that is outlawed), you will most definitely lose. And the consequences of it is that you’ll lose the capability to fight even more, since the protesters have a high likelihood to be injured, arrested, etc.
2. Because there is no explicit consensus among everyone who uses no platform strategy, you are setting an arbitrary standard about what values is “punch-able” and what is not. For example, some anarchist thinks that liberals are also “punch-able” even though liberals empathize towards leftism to a certain degree. The harm of this is that you won’t create a coherent movement especially given that contrary towards non-violent action, no-platform strategy necessarily requires you to act on the behalf of “all leftists”.
3. You are going to legitimize the hateful rhetoric from the far-right that says that leftists and all the actors they want to protect are inherently violent (ex: Rhetoric that all blacks are criminals, all Muslims are terrorists, etc.). This one is pretty self-explanatory.
The ideal strategy for the left to implement is to fight speech with speech. The left must point out the flaws of the alt-right and expose them for who they really are. Sometimes, the alt-right does a kind of speech that is legitimately universally unacceptable (ad-hominem towards trans, inciting violence against Muslims, etc.). And when they did say that, capitalize on that momentum to decrease people’s empathy towards the far-right and also to decrease their legitimacy in the eyes of government.