One of my biggest concerns is that the button is a soft power off. If you have a new desktop computer, try pressing the power button to turn off the computer. In theory, you can hold the power button for 5+seconds to turn off the computer; in practice, it could take 10 seconds or more. At 60 MPH, the car can cover a lot of ground. Worse yet, think of even 5 seconds of a tesla at full throttle! Think that's bad? Turning "off" a modern car doesn't really turn it off. So, you really can't stop it, when things go wrong. The throttle is now electrical. There is nothing to stop an electrical car, when the ehim to run strikes its fance. Well, nothing but battery depletion, that is.
I can turn off my key, any time.