Found out a car had one when I drove though some deep snow in the driveway.
I don’t see why putting it on top of the front or rear subframe would be bad. It wouldn’t get knocked off by snow that way.
Getting the door open is definitely a positive.
Not starting the car is another issue.
There are probably some ways around that. 
If your car will not allow locking the doors with a fob inside, might be able to wrap it in foil. Never tried it. We use both fobs on that car.
Wrapping the fob in foil works quite well.
The bag plastic between the Keytainer™ magnets and vehicle metal may make it adhere less.
Criminals knowhere these are likely hidden.
I hid a key in themergencyehicle’s gas door under the metal which attaches door to hinge.
No one found it.
Thelectrical tape in which the key was wrapped made it easily removed.
I have a good place. I’m not going to advertise it. A friend who worked as a cop in a small town (Maquoketa, IA) in the early '70s said his most-common call was opening the cars of people who had locked the keys inside. He slim-jimmed them. Can you do that anymore?
A few years ago I used a coat hanger to unlock a woman’s car.
Now cannot remember how I did it. Could not hook and pull the lock button.
Another time I used a key fromany keys on a key ring I have.
The key opened the door but would noturn the ignition.