That would be nice and simple if it were true, but it's not. Sometimes it fails when my face is fully visible. Sometimes it works when my face is not fully visible. It can work or not work with the same pair of sunglasses depending on... who knows? Maybe I'm just not smart enough to figure out the exact meaning of "fully visible."
Try retraining FaceID without glasses/headwear, when standing up and holding the phone in front of your face at similar height. Anecdotally, it adapts better to sunglasses/etc. that way.
Also when it fails with sunglasses, and ask for password the first time, immediately logging in with passport will make subsequent unlocks with sunglasses work better (presumably it marks that face adaptation as "correct").
That would be nice and simple if it were true, but it's not. Sometimes it fails when my face is fully visible. Sometimes it works when my face is not fully visible. It can work or not work with the same pair of sunglasses depending on... who knows? Maybe I'm just not smart enough to figure out the exact meaning of "fully visible."