The most secure way to protect personal information on an iPhone is with a strong passcode. While Face ID and Touch ID make it convenient to access devices on a daily basis, in some situations forcing a passcode requirement improves security. Forcing passcode entry is more difficult than invoking Face ID or Touch ID against the device owner's will.
Holding an iPhone up in front of someone's face can easily unlock the device. In similar fashion, all it takes is one fingerprint to render everything accessible from the iPhone home screen.