I don't see how this can be a problem. I'm a man who has a beard of variable length that literally covers my face and Face ID always adapts perfectly. I first registered my face with a full beard, then at some point, shaved my face clean. Face ID didn't skip a beat. I grow it for a couple of weeks and go back to clean shaved. It's never had a problem recognizing me.
To make it even harder on Face ID, I go from wearing glasses to contacts all the time. Again, Face ID knows it's me with or without glasses, with or without a beard, and even with a ball cap or without it. My appearance changes quite a bit. I can't see how hair is going to throw a challenge at Face ID since it pays attention to your face, not your hair.
I don’t know either. All I know is that on the whole I unlock via passcode more than 50% of the time.
Don’t get me wrong, I’m a fan when it works.