Unions tend to act as a drag on efficiency, productivity, and personal growth. Fred, the guy who as been at the company for 15 years, will always get the promotions, educational opportunities, newest technology, etc over the new guys who may want to work harder but it would be pointless because there is a preset "line" for everything.
Sure, for industries that don't require any innovation or face little globalization unions will be fantastic but for all the high tech fields and ones in which companies around the world are racing to be first to market, the ability for any worker to quickly advance on their own merits is a powerful disincentive on unions.
Unions also tend quickly towards bureaucracy, corruption, and value loyalty (to the union) over all else, not necessarily good traits in a globalized economy.
Look at police unions for example: they wield a disproportionate amount of power over what unions really should. They can shield bad officers from getting fired or even disciplined, they can close ranks and act as a drag on innovations like body cams or citizen review boards, and they fight tooth and nail against improving hiring and training practices. At the other end of course you have companies like Walmart which would rather shut down a store than allow employees there to unionize. I don't know what the answer is, but unions by themselves can many times create more problems than solve.
Didn't they also carry Obama to victory?