I don't have an iPhone, but it's not exactly alien for me to be in a situation where I've gone more than a couple of hours without touching my phone but while it is doing something important: recording where I am. (And, yes, I have fallback options, but they aren't nearly as good.)
If you're going to put in an auto reboot either make it long enough nobody will trip it while the phone is legitimately recording something or make it configurable.
Do alarms work on iphones if they are in the BFU state? I'm pretty sure they don't on my android, because it hasn't even unlocked most of the bootloader if you haven't put your pin in.
Yes. If your iPhone updates over night (as I mentioned in another comment, common time for automatic updates or just to kick them off manually) your alarm still goes off the next day after it restarts.
Very little, which is why if you enable automatic updates on iPhones they try to apply those updates at night while the device is locked and charging, when most people are sleeping. If you're using the phone it won't activate at night and will let you know that it couldn't install the update.
Only harm I could see if someone grabs their phone to make emergency call and it's rebooting or locked and in their sleepy state, have trouble unlocking it.
However, I do think 12 hour "Phone hasn't been unlocked, reboot it" seems logical security feature to add.
