The OP and @fvr have both explained very well the issue from both sides.
Yes, it is true that the union by definition helps the underachievers and harms the overachievers, creating a downward pressure on overall productivity and quality. This would not be the case with a good union that self-regulates and pushes out the bad while rewarding the good. Of course I have never seen such a union. Why? Because that would lower union dues payments. After all, we all know the union is a profit center when it all comes down.
On the other hand, it is true that the negotiation power of the many is greater than that of the individual. So anyone can see that a union yields some degree of power to the individual, albeit at the expense of some exercise of individuality. Plus there is the financial factor. It costs money to be part of the union. In theory you get more out of it than you put into it.
So there is a rationale for either school of thought. Pro and Cons. The rub is for those who are forced to participate in a union in order to work (i.e., a closed shop) when they don't want to. Are 97% of union employees hard working? Not even close in my experience. Are 97% of management employees hard working? Not even close in my experience. The main difference is that a low-performing management employee eventually will be shown the door. But the low-performing union employee gets to stay and suck more life out of the company and his/her co-workers.