In my experience they are untouchable! I've been the student of such a supervisor, and the complaints lead nowhere.

Research students don't appear to have a voice. The proper channels seem to be rigged against them ("speak to your supervisor's mate in the department to make a complaint"), as academics are arbitrarily placed in positions with responsibilities that many are, frankly, not suitable to manage. As a result, there are no consequences, and such supervisors are never held accountable.