I was in Charlie Airborne in Alaska from 75 to 79. We didn't have a choice what boots to wear in the winter. We wore what we were told to.
I've made plenty of parachute jumps in these things, I've marched, snow-shoed and skied hundreds of miles during my 4 winters up there. My feet eve stayed warm the one time it got to -70 outside of Fairbanks.
They weren't the most comfortable boots I've ever worn, but they worked.
The boots work like a thermos, there is an inner space that traps the heat between layers. Like a thermos, they only work as long as there are no holes. in either the inner or outer layers.
Our boots got tested every winter and exchanged if bad.
Beware of used (property disposal) boots. The've been disposed of for a reason.
There is a picture in the "Badassery" thread of the Air Force guy doing the hi altitude parachute jump from the ballon, he is wearing these same boots.