Advance directives never take away decision making from a competent patient. While physicians and hospitals are not legally bound to honor the advance directives, they are ethically required to uphold them when they can, and to try to find another health care facility that will honor them when they cannot. For instance, a Catholic hospital might feel ethically unable to withhold food and water from a patient in a persistent vegetative state. If that were the advance directive of the patient, the hospital would be obligated to transfer the patient to another facility that could honor the request. Advance directives do not usually cause ethical dilemmas.
Competency has both legal and medical components.