I flew in the rain last week. Got caught off guard. The Phantom did ok in so far as I was able to get it home safely, but I got all sorts of warnings along the way in the DJI app. First it reported something about a compass error. Then something about a battery cell being bad, then a gimbal error. And that was just a mild drizzle.
Once I landed it, I pulled the battery and took it inside. I left it over a heating vent for a few hours, tipping and rotating it to allow the hot air to circulate through in different ways. I was too lazy to open it up, but it seems to have worked. The next day, after a compass calibration or two (or three), everything worked just fine, including the suspect battery.
I think I was lucky. I would definitely not recommend flying in the rain.