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.
Basically throwing junior members under the bus to secure more money and benefits for seniors, often protecting incompetent senior members who rarely showed up for work.
It just killed all motivation. It turned the job into "just show up and hope that in 10 years you can be the one screwing over the new people".
Could you expand on why you believe unions can never be net negative?