Beginning with the very name, there is little to like about the Living Will (LW).
First thing wrong, it is a misnomer and has nothing to do with a Will, the document by which you specify what happens to your worldly possessions after you die. A Living Will is described as a document which states more or less what medical treatments you wish to have or not have if you become unable to make or communicate your own decisions and possibly who can speak for you in that event. It seems from this page on the Living Will and Values History Project that the term originated only in 1969, coined by a certain Luis Kutner whose motive appears to have been the promotion of the right to voluntary euthanasia.
This opportunistic piggybacking on the common and established word has caught on but confusion reigns supreme, as evidenced by the proliferation of other terms for the same thing - advance health care directive, advance directive, advance medical directive, representation agreement, mandate, authorization, personal directive, power of attorney for personal care etc.
Why so many terms? The answer reveals problem #1 - different governments have approved different versions and a LW valid in one place may not be recognized in another. Governments includes Canadian provinces, US States and countries of the world - yup, they all have their own since of course their logic and the needs of their citizens are unique. Funny how our needs change when we move from one place to another.... If everybody stayed put in their own little town the different versions wouldn't matter but go off on a work assignment or a holiday and you would need to have a whole series of LWs to paper your trail, all prepared with the help of a lawyer in each jurisdiction in order to ensure their validity there.
Problem #2 is the practical difficulty of letting everyone know who might need to know. Are you going to be able to provide copies to all the medical people who might conceivably need to have your instructions, wherever you might be?
Problem #3 is the one of changing or cancelling a LW, especially as you done what the experts advise (like Douglas Gray and John Budd in their book The Canadian Guide to Will & Estate Planning) and given out copies to your doctor, lawyer, proxy (person granted power to decide for you) and family members. You must go through a formal process, witnessed by two people, to revoke of change a LW. Forget who you gave copies to and you may have conflicting versions, or you might have people act on a version you revoked. Of course, if you stick the LW in a place unknown or not quickly accessible when the need arises, your carefully prepared LW may never get used.
Problem #4 is that your LW may not be followed anyway. The forces of medical officialdom may decide that your wishes are unreasonable or impractical and over-ride them. You may just be creating anxiety, guilt or bitterness by your family if your wishes cannot or are not followed. Can you anticipate every medical circumstance in which you might find yourself and address it comprehensively? There's a reason hypothetical questions are mostly useless.
I do not dispute the motives for making a LW - as expressed in the pdf Living Will within the University of Toronto Joint Centre for Bioethics (you must fill in a free registration to access the copy) - to gain control over your medical treatment and to relieve loved ones of the burden of decisions. (Incidentally, this version of the LW apparently can be valid in seven Canadian provinces so the multiple jurisdictions problem is somewhat mitigated.)
However, is a formal legal document the best way to do this? Wouldn't it be far easier merely to tell your family to follow your wishes and tell them your way of thinking about such matters, give them the latitude to use their judgement and reassure them that you trust their judgement. After all, who will the doctors ask, as ask they will?
If you don't trust your family, you've got real problems. Maybe then you must reluctantly opt for a LW.
The only other limited scenario in which I could see the LW being useful goes back to its origin. If you have a terminal illness so you know what lies ahead, maybe it will be useful and practical to tell others when to pull the plug.
Otherwise, the Living Will is definitely a case of the treatment being worse than the affliction. At least stop using the misleading name.