If you are fully attentive, you can correct course once you realize a mistake is being made. My Honda Civic has adaptive cruise control and lane keep, and I run into issues with it reasonably often. I'm not complaining: after all, it's not marketed as much more than glorified cruise control. And either way, turning it on is not a risk to me. With any of these features, in my opinion, the main risk is complacency. If they work well enough most of the time, you can definitely get a false sense of security. Of course, based on some of the experiences people have had with FSD, I'm surprised anyone is able to get a false sense of security at all with it, but I assume mileages vary.
Now if the car failed in such a way that you couldn't disengage FSD, THAT would be a serious, catastrophic problem... but as far as I know, that's not really an issue here. (If that were an issue, obviously, it would be cause to have the whole damn car recalled.)
All of this to say, I think we can leave the guy alone for sharing his anecdote. When properly attentive, it shouldn't be particularly dangerous.
FSD can (sometimes very wrongly) act in milliseconds. Even attentive humans have to move their arms and the wheel, needing hundreds of milliseconds. The same humans who may have become numb to always paying attention, especially if it works well enough most of the time.
But, he'd definitely risking other road users and pedestrians if that car keeps trying to run up sidewalks and cause other havoc on the roadway.