It's undeniable that using nifty technology like the fingerprint sensor to unlock your smartphone is convenient. You also probably thought that it was more secure (for one it's encrypted and it's a lot better than no password at all, which was most people's security measure before the fingerprint password was introduced). It's becoming clear, however, that there's a gaping loophole in the security/privacy the fingerprint "password" provides. While police cannot ever force you to hand over your password, with a warrant they can force you to use your finger to unlock the phone. From the Atlantic:
"The Fifth Amendment, which protects people from incriminating themselves during legal proceedings, prevents the government from compelling someone to turn over a memorized PIN or passcode. But fingerprints, like other biometric indicators—DNA, handwriting samples, your likeness—have long been considered fair game, because they don’t reveal anything in your mind. (Marcia Hofmann, a digital-rights lawyer, wrote a comprehensive rundown of the question in late 2013, when it was still hypothetical.)"
So think twice before you use your fingerprint to unlock your phone (or as the new technology comes out, your eye or your face or whatever). If you have something sensitive on that phone - whether it's text messages, photos, or your web browser history... or especially if you're an attorney with client information on your phone - you might want to rethink that fingerprint unlock feature.