What's the worst accident involving a Model T, maybe a dozen dead? Early steam boilers aren't going to be much worse either. Nuclear accidents are essentially unlimited in size. Nothing else can do that kind of country-sized - let alone it being permanent.

Chernobyl showed the potential impact. Fukushima showed that even several decades down the line things can still rapidly run out of control. All the knowledge and experience in the world isn't going to save you when something unexpected happens and things are just waiting to spiral out of control.