Cuff someone, pull their phone out of their pocket, hold it in front of their face. "Is this your phone? Oh, it looks like it is..."
How's that Fifth Amendment thing working out for you?
EDIT: There's legal precedent that specifically says you can be compelled to unlock your phone with your fingerprint, and that's not "testimonial" (that is, your right against self-incrimination isn't in play). There is zero reason to think FaceID will be treated any differently.
Interesting twist. There was advice going around for protestors to disable Touch ID as cops need a warrant for a keycode, but don't for a touch ID. Not sure how accurate that is, but presumably you can disable Face ID if you think you're at risk of being shaken down by the police.
iOS 11 comes with a nice feature to temporarily disable Face and Touch ID if you're ever in this type of situation. Tap the power button 5 times and that's it.
This actually raises an interesting question. I say it's easier to do it since there appears on the surface to be less opportunity to mess up as with touch. Sure you could make a face etc but that just seems easier to do in stealth (not seen by cops as doing so) with a finger. [1]
Edit: As only one example you could in theory program a finger that wouldn't typically be used with touch id.
