Sweating is something you absolutely would want to avoid.

Yup, lack of ventilation is the biggest drawback of a bivy bag. Excluding excess sweating (which you try to avoid), there will always be condensation in such a bag. It will make you damp. Given enough time, it may soak your clothes completely.

And since these bags are uninsulated you just can't expect them to be anywhere near warm enough by themselves. It sure is better than without a bag - but if you want comfort you need a sleeping bag and something to insulate yourself from the ground (bough bed or ground sheet). If you want comfort for prolonged periods of time (say, more than a night), you also need a shelter with adequate ventilation and/or something to heat it with (stove or fire). Or make a big honkin' fire in front of a lean-too, but that isn't practical in all circumstances (injured or lack of suitable firewood).

The bag or a perfectly wrapped heat sheet works because

• The excess moisture will not evaporate (major cooling factor, but also a potential long term problem since your clothes will get more and more damp, possibly wet)
• The direct heat transfer from you to the air is delayed (you're warming the air inside the bag, which then heats the bag which then heats the outside air - a much slower process than you heating outside air)
• What about the manufacturers claim that it reflects 80-90% of something mysterious called "body heat" or similar? Sorry folks, the "reflects 80-90% of body heat" is a scam - it reflects 80-90% of your radiation. The reflection part is real, but heat loss due to radiation is not that important, I am guesstimating maybe 5-10% of the total heat loss from a person. If those numbers are reasonable the reflected radiation effect will reduce the over all heat loss by 4% to 9%. Significant, but the two factors above are much more important.

With a bag, you can breathe out through the hole which cuts down a lot on the moisture content. You can also experiment by making some ventilation slots - but it is a very delicate balance. Too much ventilation and you loose too much heat due to evaporation and heat transfer to air. My bag has zippers at 3 edges, so I can experiment with ventilation without cutting holes. With a sealed bag I'd try to ventilate through the main opening, not cutting ventilation slots or anything.

All those things considered, I still prefer to have the option of a bivy bag. That doesn't exclude carrying a heat sheet (which are small multi-purpose items). Trying to make a waterproof cocoon out of a flimsy heat sheet - and make it STAY that way while trying to get some rest, hydration and food - just seems to be too much of a flimsy project for me and my liking.

That being said, neither bags nor sheets will be anywhere near comfortable over the night in a cold climate. But you're much better off inside your cocoon than without one.