Consider that unions are great in theory and in many circumstances have proven themselves corrupt to the core in practice.
I consider it a credit to the anti-union propagandists that we're all so quick to discard one of the very few tools we laborers have to better our lot in life.
All power corrupts. The observation proves nothing about unions specifically. If you take away power from unions it just goes and does its corrupting somewhere else.
A few unions engage in poor practices and you are happy to tar them all with the same brush.
The problem is not the unions.