As described in the README, the combination of root access and locking the bootloader has the caveat that it's easy to brick your boot partition by accidentally making changes to it. That causes the signature check to fail, and then you have to unlock the bootloader and wipe all your data to re-flash it.

I don't know if there's any good solution to this, since all this seems to be necessary for the security model.

EDIT: Wait, isn't this what A/B partitions are for? (ie, you can brick one partition and still boot from the other) Also, shouldn't it be possible to flash an image signed with the correct keys without unlocking the bootloader and wiping the user data?

It also has the caveat that protecting against privileged attacker persistence doesn't work by definition, so it only provides protection against physical attacks. The protection against physical attacks is also reduced through having the keys available on a lower security device as would typically be the case.