My problem with space blankys is that you can't keep the separation from your clothing large enough to let them work as a heat reflector.When they are tight to you all they really are doing is either keeping rain off you or breaking the wind.
They have no value as insulation for heat loss by conduction instead of radiation.
They are not really big enough to be an effective tarp either.
(the original ones with the ripstop fabric and grommets were a bit better, but not enough to matter.

In return for that they trap condensation from your perspiration and make your clothes damp if they are not wet already.

Large (yard size?)heavy duty garbage bags will do all of that for me and much more cheaply.
Orange trash bags make pretty good flags too, and might increase the odds of me getting found if there is anybody looking for me.

Trash bags are also just strong enough to use for other things too like tarp, water carrier....etc

Where a space blanket does work is hanging it at an angle above or behind you to reflect more of the radiant heat from a fire onto your body instead of losing that radiated heat to the trees or outer space.

If you are sitting up and the Space Blanky is draped loosely it is able to work better.