Much more important though is that most unions make work predictable. Hours, duties, etc. However, most PhD research is highly unpredictable. If I want to set up the test while the conditions are good, I may want to work NOW; hearing that I'm out of hours and need to do it tomorrow is the last thing I need. If I got my test set up (in shared lab) and going great I may want to go as long as I can stand it -- it may be broken tomorrow.
At least last 2 years of grad school my #1 desire was to finish and go use my new PhD in real world for real money. If union imposed policies add 1-2 years to the process I would not want them.