I've been a member of three different unions throughout my life, and I've never seen the behavior you describe. I've heard it many times from anti-union people, but I've never actually seen it. I don't see why any union would want to make it difficult to fire a crappy employee. That only hurts the union.
Like the person you responded to, I have seen more than a few managers that aren't willing to be the one to document misconduct or poor performance, or are unwilling or afraid to be the one to actually confront an employee and/or pull the trigger on termination.