It is an interesting question, and not one that I have seen addressed before. Although often outnumbered, I think the Arditi del Popolo managed to acquit themselves quite well in fighting against the Fascist brigades. It was really the state security forces that were the downfall of the group. If you can keep them secret when they are not fighting, they should manage to do a lot of harm to the Fascist power structure. Whether or not it will be enough to actually defeat Mussolini's men outright is an open question. It would be interesting if this turned into a Spanish Civil War analogue, 15 years ahead of time...