From my experience having worked with several different CNA unions, it all depends. Although unions do work to protect/support their members' interests, I have seen where several of those unions did NOT want BAD employees. It reflects on their reputations and can antagonize management/union relations. Some unions are reasonable to work with.
The disciplinary process can be very difficult but it CAN work. But it does take work on the part of line nurses, supervisors and management. And sadly, there are many points of weakness in the chain links that can derail the process.
But it can work. And yes, once a union does get in, it is usually there to stay.