I'm bothered by the move to TouchID and now FaceID from a "the government can make you unlock your phone" standpoint. Legally, the precedent is that the government can force you to use "something you are" like your face or fingerprint to unlock your phone. Currently there is at least some ambiguity to the precedents that say they can't require you to unlock your phone with "something you know" like a pin or password because of that could be construed as self-incrimination and forbidden by the bill of rights.

If you're worried about law enforcement requiring you to unlock your phone DO NOT USE TOUCHID OR FACEID.