Yeah, the whole concept of employers recognizing a union or employees being required to join a union which is raised in US union discussions are kind of weird.

Employers don't have to recognize a union in Europe, employees can still join and avail of legal support or group actions by those employees that did join.

Similarly, if the majority of your workplace is in a union, you don't have to join, just don't be surprised they don't bother defending your case if you end up a victim of unfair dismissal. Your employer may be unwilling to offer you employment terms different to what the union employees get because of the overhead for them, but if you had that little leverage you probably didn't have the imaginary invididual negotiating power the anti-union crowd imagines anyway.

Minority unions undermine the collective bargaining that underpins the very concept of a union. They make no sense to me, at all. The whole point of a union is having a large enough block of workers committed to act together that the company is forced to either negotiate or perish. This obviously works when you have all the workers unionized, as a company without employees can achieve nothing at all. As the union gets a smaller and smaller fraction of the workforce, however, their bargaining power diminishes dramatically.

That sounds like a really good argument for a union providing sufficiently valuable services to their members that they want to be a member. If a union isn't serving its members those members should be able to seek better options.

If unions worked like this in the US, it seems like many more people would be in favor of them.

Unions by and large do work like this in the US. There's fewer unions, but the unions that do exist tend to do a very good job of representing their members. The main force acting against unionization efforts here isn't that the unions are ineffective, it's that they're so effective that companies fight tooth and nail to suppress them, including spreading a lot of propaganda.

And the problem with open shops and opt-in unions is that scabs can be found who think in terms of the short term. A company, in contrast, has the luxury of playing the long game. So, faced with union demands, a company can hire a bunch of scabs offering benefits and pay equivalent or better than the union is currently fighting for, then when the union is crushed gradually fire the scabs and replace them with new workers who are back to the square 1 the union was fighting from. And because unionizing takes effort and conviction, the new lot might not unionize. The company wins.

Because unions often support policies that significant portions of the workforce do not like. If you're a teacher in America chances are your union dues are being used to oppose merit-based pay increases and retain seniority based pay. If you're a teacher who wants merit-based pay increases then you're being forced by the government to fund a group that is actively working against your own interests.

Multiple unions would let workers join the union alined with their interest.

The practice is called a "closed shop", and it was ended in the UK by Thatcher and Labour gave up on trying to reinstate it because it was unpopular. https://www.theguardian.com/politics/2020/dec/18/labour-aban...