I think all unions should be a choice. I was a forced AFL-CIO factory worker and hated it but needed the job.
I wanted to get paid on my merits rather than what the union and company negotiated for all.
I kinda like the idea of membership in The Steam Pipe-Fitters Union being mandatory for the type and quality of work they are expected to perform. The rest of the trade unions aren't as dedicated to perfection, and their work isn't as life critical.