While it's not particularly widespread, there are places in the US where grad students have been unionized for decades. The Universities of Michigan and Wisconsin are two of the oldest, dating from the 70s iirc. As a grad student falling under the collective bargaining agreement for the University of Michigan, I feel pretty good about it. Not only do they pay us pretty well and give good healthcare, but the union has been essential in making the administration set up better-than-average covid policies, like free asymptomatic testing and non-retaliation for choosing to work remotely (sometimes). There are also a ton of corner cases where subpopulations of the grad students have a specific issue, where the union has been a very good advocate.