Two facts that have been documented that I know about: 1) states with strong teachers' unions also happen to be very highly ranked for the education they deliver (for example Mass.) and 2) they have a slightly higher turnover than states with no teachers' unions. That is, the idea that unions guarantee jobs independently of performance is not supported by the facts.
Every day I open the paper, I am told that the greatest robbers are pensioned state workers, unionized workers, and any kind of public "entitlement" program -- with the exception of govt support for the armed forces, agribusiness, finance, etc.
The evil teachers unions have been hotly debated in an Oakland Unified School District chat group. One man in particular wailed loud and long about how horrible public education was because of the greedy unionized teachers. Finally, he wrote a long diatribe about how well teachers were paid. Unfortunately he estimated their salaries to be about twice what they actually are, and has been relatively quiet since. What's interesting is that he had almost no support on the list from others.
Joanna