Many unions in America have developed into malevolent forces that seek unfair competitive advances, often to the detriment of many people working in that industry.
Most of my knowledge in this domain is contextual and thus useless but here is a good example of the more extreme side of some unions. [0]
I want to make it clear that I am not against unionization. What I am against is the use of rhetoric and promises of fair treatment to create a mob-like organization that uses threats and coercion to not only manipulate the local government but the people who exist within that organization's industry.