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This may not be helpful, but I tend to join unions out of principle more than anything. Unions that are ineffectual tend to be that way because they're operating in industries with massive historical defeats, and a resulting lack money and support.
It can certainly be a bit of a sideshow with its own weird internal politics, but it's one way of lending a bit of weight to worker's rights in an otherwise authoritarian system of work.
Ironically, the times in my life where I could have most used a union, none were available because of the nature of the job. Temporary workers rarely join unions or stay around long enough to satisfactorily resolve disputes, so they've invariably been the ones with the most unreasonable conditions and miserable work environments.
Easily the strongest union I've ever paid into was when I was with the NHS. There's a few different choices, there, but the one I looked at seemed to really take care to resolve disputes and they actually had some credibility.