There really aren’t any large organizations involving humans that are free from corruption. It’s also relevant in reading things about unions that there are people with money and influence (usually business owners) who would see them as evil whatever they did. Pope Francis sees them as an important safeguard for the human dignity of workers. http://bit.ly/bc-labor2. A scholarly source like this Companion to the American Labor Movement can give you some context on why unions formed: http://bit.ly/bc-labor1
The grad student union cites a variety of Catholic social teaching in its own support: bit.ly/bc-cst. BC’s stated position is that “graduate student unionization in any form undermines the collegial, mentoring relationship among students and faculty that is a cornerstone of this academic community” That’s a quote from a longer statement by Provost David Quigley. bc.edu/bc-web/sites/unionization