Android's encryption is pretty useless, as it forces you to use the same key for the encryption and for the screen lock. So either you have a secure encryption key and a way too complicated screen lock key, or you have a reasonable screen lock key, and a totally insecure encryption key (that can be brute-forced in less than a second).
Why would they implement the feature that way? Why not have a boot passphrase? Wouldn't you need some custom hardware to extract the keys out of RAM? And Google is in the right position to mandate some extra security hardware, if needed to provide a secure design.
Not true anymore. Device encryption can differ from lock screen nowadays. But speaking of lock screen, people have built robot to brute force it, so shutting down after a few failures is definitely a good idea.
