The problem comes when unions start to jam up the works by forcing companies to pay absurd rates for what is essentially unskilled labor. It drives up the cost of the product, and people are less likely to buy "made in America" if another company can produce a product of the same quality out of china.
IMO Unions have worn out their welcome. They were useful back when employers treated employees now crap. Now they're just giving companies a reason to outsource. In my job several dumb people have been talking about trying start a union because the company had to completely stop giving raises with the downed economy.(crybaby employees) Which sucks, but people act like it will fix things and get them their raises back. Ignoring the fact that the union dues will probably eat their raise for the next two years and some asshats will regulated the hell out of a good job.