Similar to what Robert said, I'm not really sure why any freedom loving conservative would be opposed to a group of people organizing in an effort to leverage their position with their employer. That would be the literal purpose of a union. It's a free country and one freedom is that of people to organize, so hey, go for it.
But that isn't what unions are anymore. Unions are a business. Like any business they have to make money and therein lies the problem. How does a union generate increased revenues so the people who run it can make more money? They need to acquire more members to collect more dues from, but there are often geographic limitations to that, and/or they need to improve the benefits of their members to justify increasng their dues, to make more money. They have to do that reguardless of whether or not their members actually deserve more from their employers. The purpose of a union has long since passed simply seeking what is fair compensation from an employer, to constantly seeking more and more benefits that people may not deserve simply to continue lining union leaders pockets.