The theory behind a debris hut is that you create insulation around your body. The only insulating material in the world is trapped air. Granted, a debris hut will work in a nice dry environment with lots of debris which create a lot of airspace. Even so, it will take one person many hours to make even a moderately functional model. That's too long! Also, when wet, I'm sorry, but there is no air in water, and in my experience, a debris hut will only work if you make it while it is dry out and it doesn't start raining bad enough to soak it through. Once it is wet, it contains hardly any air, therefore no air space, therefore no insulation. When it is wet and cold out, there is no way that I've ever been able to stay inside a soaking wet debris hut without freezing to death. Seems to me there are much better shelters available, and that staying dry is the absolute priority in survival.
My experience with the debris shelter is in 15 degree Fahrenheit temperature with about 6 inches of snow accumulation. I tested this shelter under the conditions stated with jeans and a T-shirt and it performed very well. However, I agree with you, it took way too long to construct, took a lot of effort and would be my choice only if I could not do better and/or if there was plenty of debris to build it, easily accessible. I had about 24 inches of debris on the exterior and had the interior stuffed with dry leaves that I packed it full with and packed down 3 times before I crawled in.
TRIBE, P.O. Box 20015, Charlotte, NC 28202, USA
I have used very wet leaves once while in a downpour, and was soaking wet while making one, but when it was done and I crawled inside and took off my wet clothes, I was warm. It was above freezing for that one. I had mostly cotton on (jeans, t-shirt, wool sweater and a nylon jacket). The coldest I've been in one overnight was -10F. I didn't have the door well made and it leaked cold air the whole night, but I was able to sleep for most of the night, and make it warmer by the 2nd night. I was wearing wool pants/shirt/jacket/hat/mittens. I just used debris (the fluffiest dry leaves I can find, or grass and ferns) for inside, and stuff it completely so I have to jam myself in. It may take 5+ minutes to wiggle my way back in with the toes pointed.
After one is properly made, it will remain dry! I make mine with a min. of 3' of debris on the outside (4'+ for -20f and colder). Wet, dry, it doesn't matter much, just make it thick. I have been inside during flashfloods (6" rain over night) and stayed dried. If not made well, you will get wet. Some bark on top will also help keep it dry. I have one I made a few years back, and it still remains dry inside, even though I haven't done any maintenance on it for a year.