It always depends how the union works and what they negotiate for. Brad Pitt, Jennifer Aniston, Scarlett Johansson, Charlize Theron and Tom Hanks are all member

It always depends how the union works and what they negotiate for. Brad Pitt, Jennifer Aniston, Scarlett Johansson, Charlize Theron and Tom Hanks are all members of a union (SAG-AFTRA) and it doesn't seem to have prevented them for making 10x the money of an unknown actor. That said, there's a risk - even a high one - that Union Executives, over time, become rent seekers instead of advocates for their members. It's like government, every Union gets the executive it deserves.