Police have yet to completely wrap their heads around modern iPhones like the X and XS, and that's clearer than ever thanks to a leak. Motherboard has obtained a presentation slide from forensics company Elcomsoft telling law enforcement to avoid looking at iPhones with Face ID. If they gaze at it too many times (five), the company said, they risk being locked out much like Apple's Craig Federighi was during the iPhone X launch event. They'd then have to enter a passcode that they likely can't obtain under the US Constitution's Fifth Amendment, which protects suspects from having to provide self-incriminating testimony.
There are ways around this system, whether or not they're ethically sound -- the FBI recently forced a suspect to unlock his iPhone X using Face ID. Some warrants can explicitly offer permission to use face unlocking. Many investigators won't know about the alternatives, though, and there won't always be an option to use the accused person's face. There's no guarantee other countries will allow forced face recognition, either.
The slide also underscores the challenge face recognition poses for investigators, especially when there's no fingerprint fallback as with newer iPhones. Where it's easy to avoid accidentally triggering a fingerprint reader, you can inadvertently set off a system like Face ID just by raising or tapping the screen. Like it or not, officers will have to be much more delicate when they want access to recent handsets -- especially when they aren't guaranteed wired access.