It’s pretty strict and won’t authenticate if I look too tired, so I think you’d have to be pretty careful about how you tape the unconscious victim’s eyes (a conscious victim would just look away from the screen) to fool it.

And if someone has full physical control over you such that they can open your eyes without consent, do you really care if they can unlock your phone? Your life is in their hands at that point anyway.

You have to assume a persistent attacker with physical access will be able to crack the device regardless.

In such a situation I'd assume you have much more important things to worry about...

Wrenches are cheaper https://xkcd.com/538/

It only works if you're close, alive, your eyes are both open, and looking right at it. I doubt that degree of specific physical attack is in most people's threat model. It's only backing a 4/6 digit pin for most people anyway. Realistically, it's not the weakest link.