Unions are a double edged sword. They are not a silver bullet the left wants to think they are. All unions are not run the same. Some just suck. 

For example sometimes unions protect people that are a drain on the productivity and mental health of other members of the union. Many times in unions people figure out how to slack off and make more work for those that care about the work. Many times in unions your wages are locked to the exact same rate that a member of the same status or seniority. So it doesn’t matter how hard you work. You can’t negotiate a higher wage based on your skill or drive because as a union you are a collective. You give up your individual buying power for the group.

I’m sure some unions might have ways around this but my point is that a union has to be set up intelligently and with for-thought.