I think a lot of the HN crowd has enough experience and clout to keep from getting exploited, but I'd personally love to be a part of a large union where I can help mentor junior engineers and get help in turn from more senior ones.

I think beyond shared negotiation, what a union could really help with is managing younger engineers' careers. I stuck around in a bad first job way too long because I just didn't have enough experience or exposure to successful engineers to realize how crappy my job really was.