It tends to be the reason so many Americans are anti-union. They do a lot of good for the average worker but they also carry along a lot of dead weight that can’t easily be shed.
In the same way we have the concept of 10x, 1x and negative-x devs, other trades have faster and slower tradespeople. Anti-union American laborers usually believe that they can outrun their fellow workers while making the additional money that implies. Unions say that they're beggaring their neighbor and the end result is they will be paid the same for more work.
Most unions focus on things like seniority, which is a bit of a detriment to everything but is very reinforcing to the union. The most senior people have a lot to protect, and by the time the junior people achieve some seniority, they have invested a lot of time in the system. A union oriented around productivity or skills would have less strength as its members aged, and it would be easier to poach the high performers into a non-union position.