It can be raining pretty hard, and still be VFR. In Canada, at least, day VFR limits are three miles and 1000' ceiling. I've flown in some pretty heavy rain, but still had lots of vis.
Even in a C172, the rain runs off pretty well. Doesn't hurt to have that big fan up front, I guess. But I flew a glider in rain yesterday - couldn't see all that well on launch, but once we were above about thirty knots, the rain ran off quite well.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.