There is a lot of corruption news about the union bosses. There is even some where the staff were forced to contribute donations for flowers of the workers who had passed. They said that the top officials pocketed a lot of that money. It don't know if it's been proven yet but I have no doubt that they did that.
Where I worked it was proven that the bargaining committee was charging the union for hours of negotiating a contract when they weren't. Nothing was done about it.
The concept of a union is good but greed and corruption usually win out.