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

You can tap the turned off screen to wake up and it works from a surprisingly steep angle. (I just did this exact workflow)

My understanding is that the screen wakes up to a slight nudge on the screen, though I’m not sure how well Face ID works at such an angle.