When there is high level of radiant energy or when "delta t" (ie temperature differences) is large.
If you can hold some part of your body in front of a magnifying glass and so concentrate the infra-red coming off of it so that it will threaten to start a fire.......or even warm a tiny spot, then reflective works. If a mug is filled with something very hot compared to the room around it, the reflective cozy works. Otherwise it is easy to confuse reduced evaporation due to impermeability to vapor with heat retention. In other words: we don't reliably and immediately sense the difference between stuffiness and warmth.
Further, this is not a statement just about subjectivity. Put a non-insulative reflective foil around a mug that is just 10 degrees above room temperature, and set another mug up without the foil, and you will be hard put to tell the temperature drop between the two. The big difference in conditions is when the mug is radiating lots of heat ie when it is very hot compared to the environment, or the contrary, when the contents are very cold compared to the environment.
The reflective coated nylon is intended to extend the life of the nylon by reducing degrading radiation, including UV, by reflection. Also to reduce the pass-through of solar rays that are quite warming. Claytor sells rain-flys / tarps made of such stuff exactly for that purpose. http://www.mosquitohammock.com/ (scroll down to Sun Fly.)
On thinsulate (tm). Probably doesn't get much credit or attention here because it is designed for clothing, to be tailored, where bulkiness including loft are NOT wanted. Like pre-snowboard ski-wear, and the lining of boots. Zillions of yd^^2 of it sold to clothing mfgs every year.