Read this
If employers are abusive, that is illegal. If employers are unfair, that is tough luck.
Unions are mechanisms of exclusion, that traditionally relied on violence to gain privilege, (violence against the firm, and non-union workers competing for jobs) and now rely on government law (violence) to excluse other workers, limit labour competition and bully employers.
Yes, people can form unions voluntarily. But a union’s bargaining power is tied to privilege which is a statist tactic, not exclusion (via property) which is a libertarian tactic.
A libertarian should be obliged to utilize Bastiat’s seen and unseen analysis regularly. We see what the unions provide for union workers. Now what are the costs? Obviously, less competition, artificially higher wages, which lead to artificially higher prices, exclusion of workers without union membership, compromising the property rights of the firm etc etc etc.
In a free market, a union serves no purpose. Wages will be bid up to their market rate. But some (incorrectly) believe that the free market can’t deal with racism, sexism or “unfairness” in general, and so we have a lot more work to do towards economic literacy, even within the ranks of libertarianism.