This category is full of claims that don't hold up, so here's the honest version.
It has no temperature rating, and it isn't insulation. It's an emergency heat layer, not a camping sleeping bag. Nobody should tell you it keeps you warm down to a specific number, because there's no test behind that.
It does almost nothing against cold ground. That's conduction, and film doesn't stop it. Lie directly on a frozen floor, and the floor keeps pulling heat out of you. So put something between you and the cold surface. A sleeping pad, a car seat, cardboard, a pile of leaves. Anything.
It's water-resistant, not waterproof. The shell sheds rain and splashes. The opening and the seams aren't sealed, so it isn't submersible.
It's reusable with care, not indestructible. Aluminized film tears once it's punctured, and it can stick to itself in storage. Repack it gently, store it cool and dry, and check on it now and then.
That's the real list. Anybody selling you one of these without that list is telling you a story.