I checked the directions for the MRE heater that reposes in an odd corner of my gear closet. If I were hypothermic, following the instructions would be fairly challenging. There are several fire starting techniques that only involve large motor skills that would probably work better - and getting a fire going has worked fine for me in those times when we were dealing with hypothermia.

What these heaters provide is flameless cooking - a very good feature in a combat zone or in extremely dry conditions, but actually somewhat counter productive in the usual hypothermic situation.