Julia and I had ventilated covers at our Blue Heron hives. She left hers on all winter. I replaced mine with an inner cover. This spring hers was propolized, as you might imagine. I think the disadvantage of the ventilated cover is unlike the slatted rack, it really should be removed for winter.
Linda have you seen the insulated, ventilated inner cover
from Honey Run? First winter on my 3 hives using them with SBB and I had no condensation this winter like the past.
I want to experiment with the slatted racks on those 3 as well.