It's one of those situations where no matter how you state your view of the situation, you are a "bad guy" in someone's eyes.
For me, being pro-union would mean being supporting of a system that I believe is completely broken in its current state. I do think the idea of unionization is important, necessary even, but like with many ideas with potential, the current system does no good.
So if I come out and criticize unions, pro-union people would only listen to my first sentence before the meathead demo (and there is one for unions, as there is one in all things) would be calling for my chemical castration. The second I'd say there's a valid point of criticism, there would be people wanting me beheaded before they even heard what I have to say.
So am I pro- or anti-union? It's a trick question that leads to a response with no pleasing answer. Like global thermonuclear war, the only way to win is to not play.