Given the amount of incompetent talent I've seen promoted without a union because they cozy up to management I have a really hard time believing the picture would be any worse with them.

Meritocracy is orthogonal to unionization, I think, though I doubt there's an employer in the world that doesn't believe that their decisions are entirely meritocratic.

The argument "[institution]* tends to towards corruption therefore we shouldn't have [institution]" is a logical fallacy in all cases, since all human institutions tend towards corruption but we still need them.

* replace [institution] with government, government department, police, corporations, unions, etc. and the fallacy remains the same.