Yes.
Unions do not bypass individual performance by design. I used to pick apples in the fall when I was a kid. For every 10 good apples there were probably 1 or 2 bad ones. It's just the nature of things but you are probably right because there is no such thing as a drunk or drug addict or a lazy person in an a non union shop.
I would say they are the same whether a shop is union or not. The difference is a non-union shop will fire and replace them. A union coddles them, defends them, and pays for treatment, and the list goes on. Look how many were busted in Detroit this year at auto plants. Only by the third bust did they get let go.