When talking economics one should be careful not to taint it with politics - from either side of the fence.

Unions are a part of the free market system. Just like management has many tools at its disposal, so too does labor in the form of organized labor or a union. In a perfect world, each is free to bargain/negotiate and then agree on a contract of terms. Then the organization takes its place in the free market and is able to compete effectively or not effectively against other similar entities. The market will dictate whether the company is competitive or not.

The waters get murky when politics enters the picture, particularly when a party favors one side over the other. Instead of referees making sure both football teams play by the rules, the refs skew the game by favoring one team over the other. With that analogy you can see how the market isn't really "free" under those circumstances.

I think it is nonsense to correlate a particular political affiliation with a particular stance on unions. It is entirely plausible to be conservative and still support your union. You must look at the union, it's purpose, and it's goals from a business perspective and not from a political one. The only reason any given politician says he or she is pro-union isn't because they care about workers' rights, it is because they are using you to get your vote. In other words, unions are just part of a politician's playbook.

Is unionization bad for business? In order to answer that question I think you have to ask it in the context of a particular union(s) in a specific industry with their respective company(s). I don't believe there is a simple yes or no answer that would cover all avenues.