Interesting. I slept in a hammock in -7 degrees, without the protection of a tarp, using just a closed cell foam mat inside a summer synthetic sleeping bag as an undermat, and then lying in a winter weeight down bag. No underquilt. I was toasty warm.
I think that what works for one person may not work for another. My companion that trip used a down underquilt and was cold.
It really does depend on the person, my mate I hammock with usually only uses a mat and says he is fine whereas I was using an old synthetic sleeping bag as an underblanket and I was feeling the cold. It could also be psycological as I think of that cold night every time I get into my hammock and I suppose it cold be something deep down telling me I'm going to be cold that's actually making me cold.