You haven't tried it I take it?

It is taught by many SAR teams, the US Air Force survival
instructors, caving instructors, climbing instructors.

Works way better than a cheap space blanket. It is more
durable and can stay on in many more conditions, such as
climbing or walking.

Dozens of people have survived using a trash bag as shelter.
The candle or carbide light inside really works too. Cavers and
climbers often carry a trash bag in the liner of their helmets.
It is always there, something that can't be said for a goretex
rain suit or a VE-25 tent. One of my friends survived a long wait
for rescue deep in a damp cave with a compound femur fracture.
The trash bag and carbide lamp kept her alive till she could
be evacuated.

Goretex wasn't even considered suitable rain gear in Yosemite
for the big walls by the Park Rangers and SAR. They said they
would
fine people who had to be rescued if their rain gear was
a waterproof breathable kind since it failed so often to keep
people dry in the long term.