MiFi's should not be overheating to a point where they are failing. If you find that your MiFi is too hott to handle then perhaps your battery is defective or not charging correctly. Defective devices should be covered by the MiFi's warranty so feel free to call it into VZW support for a replacement.
Keep in mind, you should not be exposing your MiFi to extreme conditions either. Leaving the MiFi on the dash of a car, in a window well at home or inside a carrying case all day may cause the device to heat up and damage itself. You should be smart and keep it in the shade and well ventilated while the device is powered on.
For example, I can leave my MiFi 4620L on all day (charging when needed) without it even feeling warm to the touch. However, my MiFi sits on a desk and is not exposed to extreme conditions either.