A mechanic didn't replace the timing belt in my '03 like I paid him to. As a result, the timing belt snapped while driving 70+ mph on the highway. Instant complete engine failure. I had no trouble shifting to neutral, getting the car from the fast lane to the right shoulder (aside from only having existing momentum), and bringing the car safely to a controlled stop.
If you lose power in a fully drive-by-wire vehicle, you won't have any control. They're supposed to have mechanical redundancy.