I see unions like judges -- as a foundation of a democratic society.
They can both be corrupted by money, be involved in organized crime, but can also make a tremendous difference in the lives of thousands by Doing Their Job (TM). Removing judges causes anarchy (the problem they were designed to fix) and removing unions concentrates wealth in the hands of a few non-working people (the problem they were designed to fix). If we look around, union membership is at an all-time low and we have wage stagnation. Coincidence? In countries with higher union participation, you also see benefits like mandatory paid vacation, wage growth, and single payer healthcare.
People can argue whether or not union Foo is good or bad (just as we can with a given judge), but unions themselves are a necessary tool in combating the abuse of people by those in corporate governance through elections.