At first I completed Face ID set-up indoors with not so great lighting, and was getting mixed results. It would especially fail with sunglasses. I have since reset Face ID and then completed the set-up again, but outdoors with better lighting (not in direct sunlight) and I've experienced a dramatic increase in reliability. It has not failed once in the past 24 hours, and works perfectly with my polarized ray bans. Hope this helps anyone else that may be having issues.