Good question! A few years before I became a hanger, I looked into hammocks and this is the thing that stopped me from even trying one. I paid the price of that mistake by sleeping on the cold, hard, rocky, sloping ground for a few more years.
So, here is my advice.
Ditch the sleeping bag and go with a top quilt for maximum comfort in a hammock. In fact, half of my ground dwelling friends have abandoned bags for quilts. Can a sleeping bag work in a hammock. Yes, but it helps if you are a former olympic gymnast.
Cost wise, the vendors here on the forums are making top quality, premium quilts. They are less expensive then my Western Mountaineering bags, which are also in the same category. It is true that top & bottom quilts combined are more expensive than one bag. That is probably the main disadvantage to hammocking.
Warranty? You'll have to go by faith on this one, but I am convinced that you will get much better after-purchase service from one of our vendors than you will from a mass produced company, warranty or no warranty.
Drafts: make sure your quilt is wide enough for you and that problem doesn't exist. (Remember that a ground quilt needs to be wider than a hammock quilt because the bottom quilt in a hammock wraps up around you, whereas the ground doesn't).