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.