But, the obvious failure mode: you assume it locked automatically behind you, but how can you be guaranteed of this without checking? It seems to me at that point it's better to just lock it with a key, which guarantees it's locked.

I get a notification when the doors lock. I can also check the status of the lock in Home Assistant.

If for some reason the deadbolt jams, or the door was not actually locked, then I risk it for those few hours. It hasn't happened yet.

I have probably "manually" forgotten to lock my door more times than that. (e.g. Carrying items out to the car, I think I will go back for more, then I get distracted and leave instead.)

I like my auto lock feature, but I manually lock it as a habit and only rely on the auto lock as a backup in case I forget. I only have to touch the keypad to lock it and can hear it lock. Also, I can check remotely if it is locked (even though the lock can/does work completely without a need for a connection).