As others have noted, unions are rooted in socialist/Marxist theory. I think it's unfair to call what we have here in America as Capitalism. What we have is an akward blend of free market and socialism.
Both unions and corporations are rife with socialist thought. Both for the most part are in love with affirmative action. Anything that protects that awards people for anything other than the contributions they make to their entity is not right with me.
To me it's unfair to have a union where there will be "slow-downs" and "work-stoppages," while an honest hard working person can't doesn't get a job while the union protects less-productive workers.
To me it's also unfair that corporations will promote minorities to positions they didn't deserve because the government makes them, or because society will look favorably upon it. It's the same situation, someone who would be productive doesn't get the job while an entity protects the less-productive employee.
This is also why it's unfair to have lower academic standards for minorities at colleges because it brings down the academic standard at the school. It's also why it's unfair to have tenure with professors, someone else gets left out while the lazy socialist professor keeps their job.
This is why it's unfair for guys like Bobby Bowden to only recruit black skill players, because a lesser talented black player gets to compete while a more productive white player has to go to a smaller school, and have a worse chance at money from the NFL.
Anything that promotes people for things other than how good they will be is wrong, whether it's unions or caste bias.
In a real capitalist system, corporations that wasted their money on huge executive payouts would not last, because the government wouldn't protect them. A good worker on the bottom would be able to move up quickly because productivity would be valued, not seniority or diversity. The problem is that people don't always act in their best interest, and there's different motivations and incentives that people respond to that is often counter-intuitive. Check out the book Freakonomics if you are interested in that.