Ive had problems if its parked in the rain, Im not using the valley cover but it was only keep ing some water out, not all...
the most annoying part was the water all over the parts, I like a clean engine bay and the water makes everything dirty, corrodes alloy (alternator, head etc...) makes your steel fan belt pullies have surface rust form.
no really a problem when your driving, the problem is when your parked, I wash the car with the bonnet up and dont park in the open when its raining or I cover the engine under the bonnet with a tarp...