It's not available for iOS and you can only communicate with others who have it installed (which realistically will be next to no one). I wouldn't call that "perfect for emergencies".
IIRC a quarter of all active smartphones are iPhones globally. I'm not in the US, lots and lots of iPhones in circulation here. Also, no Briar on nearly all of those Androids, so they're just as reachable as iPhones.
