Every institution has a STATED purpose and an ACTUAL effect. Institutions become inefficient, corrupt, decoupled from their original goals. What keeps institutions aligned with their stated--noble--purpose, is the institution's internal incentive structure. Unions often have very bad internal incentive structures.
I would be SHOCKED if you could not find counter-examples where unions are great. It's a big round world we live on where silver linings abound. For anything. To me, right now, I do not find pro-union activists persuasive in the slightest. But I am open-minded.