I have an X right here, and it usually rests on my desk. It will unlock from a VERY oblique angle, way beyond what i expected it to be able to handle. The only failures i've gotten with the scanner were when a pillow was covering part of my face and i was in bed. Basically any position that i can reasonably read the display from that isn't a joke or a test will unlock the device if i'm glancing at it.
I have some complaints about the device, but this feature rocks as far as i'm concerned
Face ID unlock is waaaay faster than touch ID every was for me. Especially considering how often touch ID failed due to sweat / grease.