Bivy’s are minimalist shelters which have a footprint no larger than the person they are protecting. They have the advantage of being very quick to set up and have a very low wind profile.
Full Protection / Waterproof
In most cases I would not use a waterproof bivy sack. In mild to hot weather they don’t provide enough ventilation for my taste. In winter, moderate to heavy snow fall will compress any insulation inside the bivy and is likely to produce condensation on the inside of the bivy sack.
In most situations it’s tricky to get in and out of a bivy without letting rain and snow into them. Of course, there isn’t room inside a bivy to cook, arrange your gear, or do much of anything expect lay there more maybe read. Stand alone bivys tend to weight around 2 lbs. In really harsh weather I would rather carry an extra pound and get a shelter that gives me room to move and a covered access/exit. In more moderate conditions there might be options that are lighter than a bivy, while providing significantly more room and better air flow.
Personally, I want to be able to sit up fully, lie down fully extended, prop myself up on my elbows and read, and be able set up a bug free space quickly, throw gear in, dive in, and then deploy my pad, unpack my quilt, maybe change clothing and not have to be a contortionist. Bivies just don’t have this sort of room.
The only situation I think stand-alone bivy sacks shine is in the high alpine (typically climbing) where you need a very small footprint and something that is as low to the ground to minimize the impact of high winds. Maybe I am missing something, since there are people who use bivy as their primary shelter. You might want to check out The Book of Bivy by Ronald Turnbull for reasons why to use a bivy and the way to make them effective. There is an interesting article about dealing with dealing with condensation in a bivy. Black Diamond, Rab (owns what was Integral Designs), and Outdoor Research seem to make the best regarded bivys. The minimalist MLD FTK Soul Bivy weighting in at 10oz is the lightest fully waterproof bivy I know.
The classic DWR bivy is often used in conjunction with a down quilt for protection against side winds, spray, and bugs when using a small tarp. . These are often made with a waterproof sil-nylon bottom, and a DWR top such as the Mountain Laurel Design superlight bivy, and Oware Bivy Bag. The down side is that in warm weather they block cooling breezes and don’t provide a lot of room if you want a space protected from flying insects. I use a MLD Superlight Bivy.
There are a wide variety of shelters designed to protect against bugs. The lightest weight options are half length, relying on a sleeping bag to protect the lower legs and typically weight around 3-4 ounces such as the simblissity inner peace bug tent. Next up is the A16 bug bivy weights 6oz due to an integrated fiberglass pole which makes it freestanding. The freestanding nature of the A16 makes it easy to use under a tarp, when sleeping under the stars, or in a shelter. Finally there are larger bug shelters, typically designed to fit under a tarp, but can be used stand alone. Most of these shelters are made by the same companies that set flat tarps and shaped tarps.