Lack of accountability - The Union officials are accountable to no one. Their activities, policies, and procedures are self-created & self-regulated. If something is wrong, there is no higher authority that controls them. It is essentially a club, where the club leaders get to choose who gets to play in the clubhouse.

Duplication of effort - There are child labor laws. There are OSHA laws. There are wage/hour laws. The union's efforts are duplications of laws that ensure safe workplace practices. Since the laws of the land superceed any organization policies, they are not needed.

Forced Participation - If someone wants to work at a "union shop", but doesn't want to be part of the union, they are out of luck (or gonna be in a tough place). I want the freedom to pay or not pay dues. Don't just assume that because I work someplace that I am obligated to give to a cause.

Bad workers get to stay - How many instances can you point to where a bad worker gets to stay in their job because of a possition with the union. Whether its a tenured teacher who is horrible at their job, or a lazy auto-worker who is drunk on the job. Firing them out of hand is impossible. Instead, months of dealing with the union is necessary.

Manipulation of the going rate for Labor - Forcing the workplace to pay labor rates which are unsupported by output. That's horrible. The going rate is the going rate. If that worker gets "undercut" so be it. Find another job. There's only so far one can get undercut before the quality of work suffers. I'd rather play that game then be forced to pay someone what their work isn't worth.

Lack of Competition - I want the best. I don't want people who aren't the best. Union isn't the best. Union is a wildcard (could be the best, could be the worst). In a union environment, I'm stuck with whom I'm stuck with. In a non-union, I get to try a worker out. If I don't like them, I get to get rid of them and try again.

Let's start with those.
~Jeordam