Depends on what is going on around you when it happens, how long it takes you to realize that something is wrong, and what to do about it. It wouldn't surprise me if many people would not even realize that they have power assist in the first place, and just assume that the wheel somehow got stuck and there is nothing they can do.

Although I would imagine that the most common failure case is the wheel slowly getting harder to turn over a period of days or weeks.

It probably depends also on the car. I drove a friend's little Datsun pickup for a while with a failed pump, and while it was drivable, you did have to get used to using both hands to turn the wheel, especially at low speeds when you had to turn a little sideways and use and overhand grip to wrench the wheel around. Good times!

Some cars probably have more built-in leverage. A sports car may be the worst since it will have a pretty low ratio to begin with. My Camaro might not really be drivable because it has a low ratio rack as well as 285's on the front which are going to be a great big PITA to move.

For a strong guy yes, no problem, for a much lighter or weaker person it can be a huge problem. Tesla is a heavy car, it would require some serious muscle to initiate a slow turn without powersteering.

If you're driving a vehicle with purely drive-by-wire controls and no mechanical linkage, you're going to crash. As far as I know the industry hasn't really deployed that yet, but it's on the way (conceptually at least).