But when looking at a separate UQ vs. a down hammock, another factor might be versatility. You would save several oz of weight for one layer of fabric. But then- at least with that hammock, you would always be stuck with the full weight of an UQ, and maybe a winter weight one. Though I suppose that would only be a concern in the actual summer heat, since many people carry an UQ at least most of the year, if not always.
If you assume all things are equal and all the design issues are resolved, then there are 2 sides to this. A truly insulated hammock isn't capable of being vented during times where you might want to do that... but it won't vent on you during times where you don't want it to do that. There are no issues of sealing it around the edges.