You need to look into the millions union repa spent on themselves. Read up on recent scandals in Sweden for instance.
I generally don't tolerate admin-work, so I might be connecting here.
Most of the stuff you take for granted in your work, including children of 10 years old not working in factories, you had it because of unions...
>The cost of union is resources that could be spent on workers themselves.
Only without unions, nobody cares about the workers. It's then just the measly bargaining power/influence you have as an individual against some huge company, or a whole industry (or all industries combined, when speaking of basic labor laws).
Now, if you want to say some unions are corrupt, union leaders make side deals, etc, some fight for BS, yes. But that's a sign that unions are broken, not that they are bad in themselves. Historically their role for the improvement of labor conditions was tremendous.
Unions work, but its members must hold it accountable. Shareholders hold management's feet to the fire, why would workers not do the same?