People who have never been in a union usually don't know what to think about them. If there is a union in place in the public sector, there is usually a reason for it. In the private sector the same is true but there are totally different paths.
Plumbers, pipe fitters, electricians, carpenters are skilled trades. These skilled trades have apprenticeship programs. A young guy lucky enough to get in an apprenticeship program has a progressively intense course of study along with on the job training to eventually gain his journeyman's card. This says that he has the knowledge about his trade to perform the job with little supervision. The next step is Master.
A lot of times, unions didn't start out as unions. The OEA is a good example of that. It was a professional organization for school teachers. Local groups began affiliating themselves with the OEA and soon were representing these groups in labor negotiations.
There is one thing for sure, no union is an better or worse than the best or worse person in it.