Kept 2 different minis in my center console for a total of three years. I didn't have any problems with either, even during 105+ degree summers in Texas.
Not trying to be a wording nazi here, but leaving anything of value in an unattended car is never a “good idea”. It’s just going to be varying degrees of bad idea.
My iPad Pro 9.7 lives in my SUV. My SUV is black and I am sure the temps exceed the operating temp at time but no hard has been done to it. My previous field iPads have had the same treatment and never seemed to suffer harm.
I occasionally leave my iPad in the car, but never in the summer. When it cools down, it's nice to take my iPad with me every time I go out and grab it out of the car if I ever need it.
The heat is significantly less of a concern than someone breaking into your car and stealing it. Unless if you regularly drive in e.g. Death Valley I suppose.
I carry mine in the pannier of my motorcycles. For sometimes 8, 10, 12 hours in a day it lives in waterproof aluminum box in the baking sun. IPad and 13" MBP. It hasn't managed to kill either one yet. Have not so far noticed anything unusual.
It kinda lives in that right hand box while traveling
Never for a particularly long time, but I take my iPad to work with me in my laptop bag, so if I stop by anywhere between home and work I leave my iPad in my laptop bag in the car.