It's not an unmitigated good - unions are just as capable of becoming "another boss" as any other human institution. Simply supporting "unions" without asking "what kinds of unions" is opting people into being harmed coming and going.
Unions are good inasmuch they allow labor to bargain with capital by underwriting demands for better pay and conditions with the threat of work stoppage.
If unions aren't doing that because they are compromised or sclerotic or whatever, then they aren't much use. Who would argue.