I don't think unions can never be net negative. I'm a huge fan of unions in fact.

But to a lot of people who oppose them, it's a more convincing statement that they're a balancing force for good than anything else I've come up with.

I usually say, "The only thing worse than a union is no union." because I'm aware that they can have negative effects. Police unions come to mind. I still wouldn't ban them though. Police unions should really be an example of how powerful they can be for the working class, if nothing else.