Right wing terrorism poses more of a direct threat but you will be hard pressed to find anyone who doesn't condemn it. Antifa on the other hand pose more of an indirect threat because their actions are supported by a large amount of people who don't realize the danger that lies in normalizing political violence. Criticism against antifa is usually met with the response 'if you don't like antifa you must like fascism' but the truth is people don't like a group of vigilantes playing judge jury and executioner. Innocent people always get caught up in the violence and justifying political violence against one group just leads to violence against other groups being justified. If you think people should be allowed to punch a nazi then people are going to think they should be allowed to punch a communist etc.