I'm very curious to see what the long term effects of unionizing will be in Academia. I'm hopeful and optimistic it will improve things in the long run. Yes there will be some unintended consequences but academic life is already very rough if you're not tenured.
Academia likes to cast itself as a true meritocracy. If you work hard and publish good papers you'll do well. In practice it's more of a gerontocracy where senior members can single handedly destroy a young investigators career. (It happened to me and I saw it happen to several people). There is little to no recourse against this.
Arguments against unionization from already tenured faculty usually sound like: "If you want to be the best in your field you have to work harder than everyone else. Forming a union won't change that fact and may even discourage students from working so hard. This might interfere with their career goals."
However, the power structures in academia are heavily skewed and toxic. Individually graduate students have little recourse when they are wronged. Life in academia is hard. You have to work long days, the chance of success is very low. At every stage the probability of getting that next job is very low.
A lot of the field agrees that something needs to change, but no one can agree exactly what needs to change or how to get there. The people with the power are the tenured professors who "won the lottery" and are out of touch with the problems they are causing. They aren't going to be the mechanism of change even though they hold all the power.