Maybe we come from a different perspective.
I started with the Spandex, which is almost as stretchy as a pantyhose, easily stretching 100% or more. The Winterfetish Darlex Sleepsack on the other hand, I would rather compare to a thin neoprene dive suite, maybe stretching 10% if at all.
The Spandex was so stretchy that I could free my arms, and once they are free, you are easily out. No chance with the Darlex.