Prior agreement
I do let my players know in advance that I reserve the right to use UA to assist in adjudicating if the core books do not have suitable means of determining a rules quandary.
So do you implement the mechanic but not tell your players (which sounds complicated) or do you like the concept and would want to implement it, but don't?
As I said before, if it works for you and your players, awesome. If not for everyone, I'd advise not including it.
My personal thoughts on Facing is it's a neat idea, but I don't need any more complexity to my combat than we already have, so I'd not be into it. I agree it is more realistic, but I prefer only a mild amount of realism in my D&D. Each person can flavor to taste.
I do let my players know in advance that I reserve the right to use UA to assist in adjudicating if the core books do not have suitable means of determining a rules quandary.