Easy to circumvent using an app which recognizes signs the phone owner might be sending with his face when the phone is put in front of him, just like if someone says something while blinking an eye.
Ok, blinking eyes are easy to notice but I can for example dilate very easily my nostrils or fake strabismus, two gestures which would be next to unnoticeable to a human not paying close attention, but what about a vision algorithm "sniffing" the camera data flow and acting if one sign, all of them or both in a given order are performed?
FaceID should have a lock-out gesture or face, so that if someone coerces you into unlocking the phone with your face, you can use a preprogrammed gesture to disable FaceID until the passcode is entered.