I was also concerned about the hood's vulnerability to water getting in the engine compartment and I was assured by a couple of people that in a heavy down pour, there should be minimum water getting into the engine compartment.
They said that, it appears to drain away from all electronics and if any thing, maybe a little (minor) wetness at the belts. They said there is no need for concern.
Down here where it's hot, an extractor hood is a necessity but sometimes when it rains, it could be pretty heavy.