I think it depends on the goal. If I'm trying to stop a corporation from doing something profitable a large union, one that contains most corpo workers, including the ones producing this profit, can strike, halting the production that generates this profit. The union could do this for a moral reason. If the union however contains for the sake of argument 1% of the workers and none of the ones doing the work in question, then staging a protest can't force a stop to the morally reprehensible production. It also makes this 1% an easy target to get rid of thus making it harder to organize more workers needed to stop production. So if I wanted to gain this power over the corpo, I would probably protest outside of union capacity.

E: They're already gone..