Alas, I'm not sure a union can begin to fix anything except a few cosmetic things. The biggest problem is the massive oversupply of people who want academic careers versus the number of real jobs. The individual unions at the schools can't tackle this oversupply at all because it's caused by the schools acting independently. The natural game is to protect the insiders by restricting admission like many of the other unions like the AMA that actively campaign against too many doctors.
Indeed, the unions risk the bad optics of campaigning to reduce new admits in order to maintain opportunities for those already in the pipeline. In other words, slamming the door shut on people who want to learn all to restrict supply of work for the privileged.
I suppose the unions might be able to help in the rare case of a real jerk of a professor who mistreats her students, but I think most professors are smart enough to play passive aggressive games instead. The schools nurture the ability to act passive aggressively.
The rest of the time, the students in the union are going to waste their time arguing about something instead of working on their dissertation, the one thing that will spring them from the jail.