Someone in this thread earlier told me about the facial recognition improving every time you need to enter the PIN, so I've done it ever since. Not actually sure whether it has improved... I don't have any solid metrics or anything. I'm definitely going to try re-calibrating Face ID to see if the rate of false negatives goes down.You should definitely enter the PIN whenever it fails to recognize you as that's what trains and improves Face ID. I think Apple should more clearly communicate this to users rather than burying it in a nerdy support document that nobody is gonna read. I had a lot of Face ID failures at first but after entering my PIN it's now recognizing me nearly flawlessly.
The head-on-pillow thing is where it fails most often for me too, probably since your face is partially obstructed / changes a bit by laying on the pillow. Entering my PIN has helped a lot there too, but still not perfect. That's one situation where Face ID just doesn't work as well, just like there's certain situations where TouchID doesn't work well (wet and damp hands after showering/washing hands, greasy fingers after eating, wearing gloves, etc). Personally I ran into far more of those failure situations with TouchID than with FaceID, I much prefer FaceID overall.
Interesting point on the gloves! I rarely wear gloves (even though I should because DC winters suck), so I can see how Face ID would be an absolute blessing for glove wearers. Also, I hate touching things with dirty (e.g., greasy) hands and getting them dirty, and I try to avoid letting people touch my phone... so that wasn't an issue for the fingerprint-based scanners for me.