- Rampant corruption (i.e. Teamsters in the 60s is the most egregious example, but probably even a lot of current unions are rife with corruption)
- Racketeering (i.e. like what happened when Theranos tried to move offices)
- Protecting bad/lazy workers (i.e. impossible-to-fire poor performing employees that do nothing yet collect a paycheck every week)
- Blocking strategic decisions in the name of protecting employees (i.e. "Sorry, we won't let you automate X; too many people would lose jobs")
I see how unions benefit workers in the case of terrible corporations that exploit workers, but as a small business owner I'm afraid a union would morph into a parasite that hinders the business and engages in all sorts of undesirable activity (in the case of industry-wide unions).
Like I read a few years ago at some convention in NYC some organization couldn't get their booth up and running because they weren't allowed to plug in their own equipment because "that's union work" and they had to wait for a card-carrying union member to come plug in their TV or something ridiculous.