My old bug-bivy design had a 30" long foot panel of Momentum, to protect from rain spray from the back of my tarp, with no-see-um netting elsewhere, and a silnylon bottom. I used this about 100 nights last season. Whether the Momentum is highly water-resitant or not, I can't say, because at the last minute I added a back panel to my tarp and that provided plenty of rain protection. But I did find that all the condensation that wasn't trapped in the foot area of my sleep-quilt ended up being trapped in the Momentum of the bivy, so that this foot panel was consistently moist in the morning. By contrast, the no-see-um netting, which is made of polyester, traps zero moistures and the silnylon was usually dry, unless the ground happened to be wet when I was setting up the night before. Because the rest of the bivy was dry, I sometimes got lazy and didn't bother to sun dry the momentum each day, and every time I did this, the result was mildew in the Momentum. My current bivy design eliminates this foot panel, in favor of pure no-see-um netting on the top.
Obviously, the bivies discussed here are not bug-bivies, and so replacing Momentum or whatever with no-see-um netting is not an option. But you should be aware that you will have to sun dry these Momentum bivies every day to keep mildew away. Because Momentum is nylon, it absorbs quite a bit of moisture, regardless of how well the DWR finish repels liquid water, and so drying takes longer than with something like EPIC, which is silicone-coated polyester and hence absorbs essentially no water. On a sunny day, drying will be instantaneous if the color is black. But if you hike in an area with logs of fog and overcast skies, then drying won't be so easy and you better learn to like the smell of mildew.