I have one. I like it. I don't recall ever having moisture problems. I wedge my inner covers up a bit in the winter and I also have SBBs which are probably helping.
The plastic is probably like a metal roof on a building, if exposed directly to the air. at night when it gets below freezing, if there is high humidity, frost forms on the underside, then when the sun comes out it melts, and starts to dripping.