Its the users fault, I know some people who were too stupid and pulled the power out when it was updating, all because they wanted to play their stupid game.
Sometimes it happens on accident though. Mine was an accident. There are all kinds of ways to have power loss. In my case, I unplugged something else on the same power strip. It caused some kind of power spike, and the power strip turned off. How was I supposed to know that could happen? It's never happened before.
Anything could happen though. There could be a brownout caused by anything. There could be real power loss to the whole house, ie, blackout. A circuit breaker could flip. Our crappy dryer that the landlord won't replace flips circuit breakers all the time.
It shouldn't be able to brick due to power loss during the frickin download stage. Download should be safe. They should download the whole update before applying any of it. Mine bricked halfway into the downloading stage. And the way it installs should minimize risk of bricking. The thing should be able to at least show an error message, not just sit there black.
You know how phones have download mode? It's really hard to mess up download mode. If you mess up your OS, you can use download mode to fix it. Modern consoles should do that.