I finally convinced myself to setup face id on my IPhone.
However, I'm just wondering if this is secure or not as opposed to using only a passcode?
This is another one of those ostensibly "technical" questions that overlaps with legalities.
Grading Apple's tech on the basis of statistical uniqueness, precision of measurement, fault tolerance and error correction <deep breath>... YES, it's as secure as a strong passcode. But, remember, iOS has a passcode anyway, the use of which cannot be disabled. Biometrics and passcodes can both be weakened/attacked by various shenanigans.
In the U.S., a passcode is considered a form of intellectual expression, and is therefore protected by the First Amendment. Even in an extreme criminal case, it would take an impractically aggressive court order to force disclosure - with no certainty of efficacy (though a particularly lawless administration should certainly be expected to try it, just to see). MEANWHILE, biometrics are not considered a form of intellectual expression, and therefore
not constitutionally protected.
The solution is to enable Apple's extended lockdown/stolen-device features (which can be a PIA in daily use). Biometrics can be used for convenience - as long as you're safe and secure. Then, when you see police or military, or go to a school, airport, hospital or courthouse, or even suspect someone is creepin', engage lockdown to require the passcode. And make that passcode a looong one - at least 10 digits or several words mixed up with numbers/symbols. And keep your screen pristine clean - or smeared up your screen with grease and dirt - at all times, to make it harder to trace your key-presses.
HOWEVER, that level of security will make thieves/attackers/captors furious, and further assault/battery/retribution should be expected, particularly when "civil authorities" are complicit in the crime/invasion/arrest. However, you should have warned your family/friends not to respond to texts, or to phone calls in your voice, pleading for help. You'll be on your own, but at least after your remains are left in a ditch, the device might still troll them a little longer. <Civil authority shakes device harder, "Derrr... how come secrets or coin bits not fall out?">
