Unions don't exist for the employees' benefit, they exist for the union's benefit. There's a big ol difference, and because of their protected status, unions get away with a lot of really horrifying behavior.

I've worked with several unions in different industries, and I can say pretty categorically that unions are brazenly unethical.

Unions may sometimes improve working conditions and pay, but only in the sense that the mafia reduces crime. You have to start out from a truly horrifying place for their bargain to be any better, and in taking that deal you're giving up on ever having a healthy environment.