I performed my original face scan without glasses, worked perfectly put on my "reading glasses", worked just as well - put on sunglasses (very dark or reflective), didn't work, but just had to disable 'Attention'. This was all 20 minutes after opening my X, and since then, FID has been incredibly accurate.
Side note: since we're talking about FID and having your face partially obscured - the other night, I put up a hood, kind of pulled it down around my face, put a bandana around my neck and pulled it up to the midway point of my mount and it still worked incredibly well. Pulling it over my top lip (or higher) and it didn't but at that point, my face was about 75% covered, so I wouldn't expect it to.