My experience bivy camping has been limited to tarping with a water resistant/breathable bivy, but there have been some nights where even my momentum90-topped bivy couldn't handle the condensation and I got wet from the inside. I'm still not 100% sure on how to prevent this from happening. Some conditions it might just be inevitable, but the higher I am on a ridge and the farther from water, the better, it seems.
Also, there seems to be a big difference in the amount of dew that settles on my bivy top when I camp under the stars and when I use it under a tarp, under trees, so if you're using an eVENT bivy as primary shelter, and it's a humid enough night, be sure to pull the bivy closed or the insulation around your head will get wet. Then again, the moisture from breathing can condense and turn into water drops inside the bivy.
So, maybe the bottom line is try it out. If you're not 100% about the climate allowing you to stay dry, then make sure you've got enough synthetic insulation to stay warm at night if your down does wet out.