"It is an obvious and well-known fact that if you are in danger of getting fired for not being able or willing to do your job, you just file a union grievance and you have instant job security" is entirely untrue. A union contract almost always contains clear instructions on how to take disciplinary or performance-improvement actions. The key is, the action must be justified and the procedures must be followed. If they are, and the employee wants to then file a grievance under the negotiated grievance procedure, the union will normally feel obligated to allow him to do so. The biggest mistake a manager can make, other than taking action without following all of the rules, is to throw up his hands and give up when the grievance is filed. If the original action is justified, and all the rules are followed, management will almost always win.