Is it waterproof? Almost all electric boards aren't, which doesn't make them something you cannot rely on. It could be sunny when you go out and raining on the way back for example.
* bearings will get dirt in them that will greatly reduce their life * if you don't have expensive ceramic bearings you'll want to pull out the bearings and make sure to get the water out and get them dry, otherwise they might experience some rusting (even the stainless steel ones). * Stopping distance is way way longer when its raining.
If you're riding in the the rain you're greatly limited to fewer conditions under which you can ride and any time saved by riding will be offset by the time spent drying, cleaning and maintaining your board.
Stopping distance would be the real problem for an electric board in the rain, though.