A union offers other protections. Right now, your employer can (I'm assuming) just fire you for whatever they want. If you have a union and a proper contract, then they need justification and the union will back you up.
Given how teacher's unions regularly protect sexual predators from firing, I'm not sure I want the same institution in tech protecting incompetent, malicious and/or predator co-workers.
From startups to mega corporations - sexual predators and outright rapists are systemically protected. (I am aware of a few "hushed up" incidents personally here in NYC)