This feature is really fantastic, and it re-affirms my commitment to using Apple devices due to security in preference over Android. The only thing I could see that would be a superior alternative could perhaps be something like Graphene. Already today I locally set up a profile via Configurator in order to ensure that my phone can't be hijacked by some local attacks, the work that is happening Lockdown is even better and I'll be enabling this as soon as it becomes available to me.
Personally, one of the major things that lead me to start investigating Graphene and other alternatives was the on-device scanning. Fortunately, it seems Apple has backed away from this position (at least for now), and unfortunately we live in a world where if you need a mobile device there's a dichotomy. Given that, I think sticking to Apple is still the best choice for me right now, and aligns most strongly with my threat model.
I travel internationally a lot (or did before the pandemic) and my primary concern is when crossing borders. I always power off my devices to prevent warm-boot attacks, and take other precautions, this lockdown mode would be a big win in this case. Even with the other protections Graphene offers over regular AOSP, I am concerned that Android doesn't really provide meaningful ways to prevent attacks where the adversary has physical access.
