I live in a rain forest and bell boxes are used all the time here. The only time I usually have a water problem with a bell box is when they are put where they are subject to damage. The blank covers for them are not all that structurely sound and if you remove the cover you are better off replacing it with a new one because the gaskets gets stuck to the box and rip.
Another problem with them is the box holds a device like a recept and a cord is left plugged in so the cover can not close which allows moisture to get in. This is why closed while in use covers are required.
Both problems are the result of improper installation and use, not design of the boxes.
