Free first aid advice from a lawyer in the armed forces: good luck unlocking your phone after a head/face injury. Hospital will be so much more fun with your phone locked in emergency mode because it wont recognize your swollen face.
And you can disable the biometrics with a simple key combination.
As for if injured: go with something like a pin or pattern that doesn't require any paticular body part. My fingurprints never scan properly after swimming or rock climbing.
https://www.macrumors.com/2017/08/17/ios-11-emergency-sos-di...
Good luck if the cops assault you from behind or early in the morning...
Turn off Auto Call
When Auto Call is on and you try to make an emergency call, your iPhone begins a countdown and sounds an alert. After the countdown ends, your iPhone automatically calls emergency services.
Here's how to change the setting:
Open the Settings app on your iPhone.
Tap Emergency SOS, then turn Auto Call on or off.
If you turn off this setting, you can still use the Emergency SOS slider to make a call.
The pin is required to enable biometrics, it’s not one or the other.
If you think that members of law enforcement aren't going to use "rubber hose decryption" if they really want access to your phone, you trust them a lot more than I do.