TouchID has a single failure mode (and a half) that isn't that common. Wet / dampness. Solution, dry your finger, try again. Gloves are the 'half' mode as what can you expect, you can't access the finger.
FaceID has no less than 3 FREQUENT failures and a myriad of other smaller less frequent ones. Occlusion, distance and light.
Occlusion - Have your face resting on your palm... fail. Have a hat on, fail. Have a mask on...
Distance is an interesting one, but for those of us with terrible eyesight, it is constant. Phone call / text at night? Put the phone up to my face to see... FaceID fail (too close). I have to move it back away so I can't read it to unlock... hoping it did unlock before I move it forward again to read it.
Light - This really shows up when outdoors in the sun. The IR sensor gets washed out and can't work. It isn't that bad, but it does happen frequently enough to notice.
The other thing that is SO annoying about FaceID is that is has no idea WHEN you want to use it... see a face 'I must SCAN IT!' Oh, that is someone else, now you are locked out and need your password. Pick up your phone, SCAN! Oh, that is your pocket, now you need your password. The latter can be turned off, the former, not so much.
I ALMOST bought an 8 instead of a Xs when I finally upgraded, but OLED pushed me over the edge. I do love the screen, but I scorn FaceID daily and was hoping for TouchId to return to the 12. Fingers crossed for the 13, lol.