Comment by CryptoPunk

Communal support (e.g. telling your friends about your work situation, and receiving advice from those who are informed) and legal mechanisms like lawsuits already ensures workers are by and large receiving the best the market can offer them.

The effect that unions have on ensuring employment contracts are not being broken by exploitive employers pales in comparison to the effect they have in restricting the labor market by giving unions what effectively amounts to government-enforced monopolies over various work units, which lets unions exploit companies.

There are numerous alternatives to granting unions enormous extra-contractual powers over employers that are available to society to prevent employer violations of labor contracts.