We've never had an issue yet. The vents are even cracked open all winter in Connecticut to offset mold and mildew. As soon as the snow is off of the roof we're breathing fresh air!
We've had them on every camper we've owned since 1974! Probably the best money we've ever spent! The power vent covers also add airflow while occupied, even with rain sensors, installed on your stock set-up you'll still have power ventilation thru the storm, because your power vent doesn't know it'd raining.
Steve & Jan, Ava & Emma (Mini Schnauzers):
2016 F350 Lariat 4X4 Powerstroke CC/SB "PULLTOY V"
2013 Alpine 3535RE "MAGIC CARPET IV"
Proud Navy Vet!