What are some concerns professors at a research institutions might have when graduate students begin to form a union? I'm curious to ways professors might support or resist efforts by gradate students to unionize. The only reason I can think of, is the concern that grant money could hire less students if they were paid more. Is this a valid concern?

What if the reasons for unionization aren't about pay? Are there any other reasons professors might not want a union forming? Are there any reasons why professors might prefer to work with a unionized student body?
There is a study on the effects of graduate unions on faculty-student relationships:

https://journals.sagepub.com/doi/abs/10.1177/001979391306600208

As they state in their abstract, "Unionization does not have the presumed negative effect on student outcomes, and in some cases has a positive effect....These findings suggest that potential harm to faculty-student relationships and academic freedom should not continue to serve as bases for the denial of collective bargaining rights to graduate student employees."

One explanation for how a graduate union can improve faculty-grad student relations is because a union allows graduate students to offload certain bureaucratic issues that may typically be dumped on their advisor.  Suppose, for instance, a graduate student has a labor-related issue of some kind (e.g. being assigned an unreasonably high number of students).  In the case where the department or undergrad chair is unsympathetic, the grad student would typically ask their advisor to fight the issue on their behalf.  In an ideal world, this isn't the sort of thing that advisors should have to deal with, but often advisors become a sort of catch-all advocate for their students in various respects.  Were there a dedicated body such as a union for dealing with labor-related issues, the advisor's relationship with their graduate students becomes more solely academically focused.