The UIC grad student union is pretty effective at keeping their teaching hours to under 20 hours per week (so, you complain to them when there's too much grading). But the stipends there are among the lowest of good math departments, certainly lower than many non-unionized places. How much of this is due to the union being ineffective vs UIC being broke and poorly run I don't know. [Source: I used to teach math at UIC]

Overall I am not really convinced grad student unions really make much of a difference in practice, at least as far as everyday stipends and working conditions. I am at a non-unionized place now and we don't work the students any harder. But in principle it's certainly a good idea, and I have indeed seen the union be a useful advocate in certain special situations.