I'm not sure how much impact a union can have on anything that isn't purely on the "work for funding" side. I'm all for TAs to unionize with a national org because it doesn't run into this issue and there is also less overall job heterogeneity there. But how do you negotiate highly personalized degree requirements as part of a labor union? There could be some high level policy changes that would help future students, but I don't think we know exactly what would work best. And will these national orgs even choose to prioritize that kind of improvement?
The UAW unions that have formed at a few other places over the last 5 years have helped raise salary/benefits a bit, but there has barely been discussion of fundamental changes that could help fix academic training. It's pretty focused on basic labor rights.
Maybe it is just my department but most people I know wanted to do a PhD knowing the terrible salary, and what they really feel ripped off on is the lack of academic freedom that was promised as part of this training. People want a union because they feel ripped off, I get it, but the union leaders are working towards a pretty different type of resolution from everything I can tell. Maybe this is just the first step in the process, but I'm skeptical.