[MetroMPG]

Maybe she just couldn't articulate why it bothers her?

I think I could guess: an engine that stops running unexpectedly? That's a BAD THING (TM) -- I can see that putting fear into a person who doesn't understand the feature or is generally nervous about their safety.

In a lifetime of driving conventional vehicles, an engine that stops running unexpectedly indicates the beginning of a stressful, possibly dangerous and probably expensive situation. It may be hard to overcome that.

Another source of unease: you want to scoot across the intersection but the engine is currently off? That's another BAD THING (TM).