The main difficulty that many have with unions is that they become as powerful as the business which employs their members and can then negotiate contracts that can put the employer (who has to then balance the cost of having union employees and trying to stay competitive with other companies) out of business. It's the difference between a symbiosis and a parasite. Unions themselves are neither "good" nor "bad". They can be of immense benefit to workers, employees and the public at large. Until they hunger for something that their host cannot provide without dying itself. Another example of the power they wield (which some have a problem with) is that after Hurricane Sandy, utility workers from out of state who were coming to help reconnect power and restore services to the victims of the storm, they were stopped at the state lines and told that they could not assist because they were non-union. Recovery and restoration of vital services to the entire public was prevented by unions who had a stranglehold on "who can and can't do what". And unionized federal employees? Even FDR saw the inherent flaws in that idea.