The worst is that with the secrecy about compensation you don’t have enough information to negotiate effectively whereas the employer has full information.

When you join the union, you lose the atomic individuality. I've been a union member and a manager outside of a union. When I was in the union, they tried to prevent me from getting paid more - they wanted me to fall in line with the collective agreement which specified my hourly rate. If you don't go with mob, they call you a scab. Not pretty. https://en.wikipedia.org/wiki/Strikebreaker

There's a difference between unionization and trying to get board representation. Also, in many states, you're stuck dealing with a union in many jobs. You now have to negotiate with the union as one atomic person...

The thing I personally hate about unions is their political advocacy. The idea that a worker is stuck supporting a politician he might not agree with is one that makes me very uncomfortable.

Also, I think appleiigs summarized it better than I did, with the lack of individuality.

I can already negotiate better than my coworkers. Why should I tie my compensation to theirs?

Because if you and your co-workers all negotiate together, you have more bargaining power and can extract much greater concessions.

But it also means that the people of lower ability whether technical or negotiating get more but the people with more ability get less.

Unions really fight against performance based differences...

https://www.nytimes.com/2019/02/11/us/denver-teacher-strike....

https://www.huffingtonpost.ca/michel-kellygagnon/union-perfo...

As it turns out, union contracts don't just set minimum prices that employees can be paid; they also tend to set pay ceilings that employers cannot exceed without the union's permission--and that permission is rarely forthcoming. Paying the most talented, hardest-working employees more than the average Joe who barely works to rule is not allowed, you see.