Unions work best when you have 3 qualities of the job: a feeling that you cannot be better than someone else (that is quality/expertise don't count past a short training period); a uniform wage scale (that is no way to say one person is better than another); and enough skill that you can't be easily replaced.

Note that the above is how they work best. Unions do work in situations lacking some of the above, but because of that lack there is incentive for someone to leave the union and so they have troubles they must address somehow.

The above mostly lifted from https://pedestrianobservations.com/2016/07/16/public-transit...