I think it depends on who you ask and what purpose that person thinks Antifa serves. Antifa is not a homgonous group and thier individual beliefs differ between them, members range from Anarchists to Communists and everything in between. They band together against fascism while some of their tactics would be considered fascistic in nature by many. Of course not all members are violent , some have helped others during periods of violent clashes such as in Charlottesville. Most notably Cornell West was interviewed on Democracy Now describing how Antifa saved himself and others from a group of Neo Nazis that were aggressively approaching them and considered their lives to be in imminent danger. Does that give them carte blanche to initiate violence themselves? Two prominent figures on the left; Bernie Sanders and Noam Chomsky have condemed their violence and expressed the negative effects it has on the movement on a whole.
This is not about Neo Nazis or the KKK vs the rest of us, of course these groups, their message, and actions should be and are condemned by a majority of the populous. I'm also not using the "both sides" argument here, the point of the "free speech" rally was to spread their hate and kill someone with a car (there was a leaked chat conversation between the group discussing the possibility of doing this). But Antifa did initiate some of the violence at this rally. They also initiated violence and both Berkeley rallies, chasing people down trying to attack them, bashing someone in the head with a bike lock, attacking a father and son who were Trump supports, and pouring water on a man in a wheelchair. How does any of that help their cause? What will the populous say when they see individuals dressed in all black beating people for no reason? Still with all of those incidents I see some on the left talk about Antifa in a dogmatic way. I think the main issue here is that all members wear black bloc fashion which makes it damn near impossible to separate members who are violent from the ones who aren't and as usual us humans like simplicity so God forbid we use nuance when talking about the group to not conflate the juxtoposition. This dress also makes it harder to spot government infiltrators (yes they still exist), and their purpose to support or initiate violence themselves. The question is though - why won't legit members of Antifa stop these infiltrators and instigators when witnessing them participate in violent acts? Because they don't have much of an issue with it and think the ends justify the means. As Tim Pool says, would you want people like that to have power and run the country? No! That's bad for everyone.
The main consequence of their violence aside from hurting people is PROPAGANDA for the opponents they are most against; Neo Nazis, KKK, White Nationalists, and the government. Let me break this down - Want to recruit and brainwash some new members to former groups? Show them multiple vidoes of the "other side" beating supporters of the President they support and love, just that simple. As for the government; they want the violence to happen too, look at the way the cops stood down while both sides were fighting each other at the Charlottesville rally and I'm not buying that it was for their own safety because that is never a consideration when it comes to "black protests". Their purpose is to stoke fear amoung us citizens so we are more comfortable when they take even more of our natural rights. I already consider this country to be a pseudo police state and it will be more prominent when the populous is afraid to go outside because they don't want to be assaulted by someone wearing all black.
Non violence is always the answer, except in the most extreme cases such as self defense, etc. Causing violence in any other lesser situation is just counter productive to your cause and not to mention immoral.