Unfortunately, they’re mostly cons. The idea of covering a sleeping bag with a waterproof/breathable shell fabric is appealing because it would mean that you don’t need to carry a bivy sack to sleep in a snow shelter or worry about getting internal condensation on the outside of your sleeping bag when you touch your tent’s walls at night. But experience has shown that covering the exterior of a sleeping bag with a waterproof breathable shell tends to trap more perspiration inside the insulation of a sleeping bag than one a much lighter shell fabric.