Rain is distilled water, so basically insulator, but still, it can be contaminated and can transfer small current, so it can do something on base of "signals".
Motors are not problem at all, they will run in water without problem.
The danger thing is switch of any kind. I like positive switches - if shortened, model will finish flight, danger comes at next battery connection, if shortened, model will beep and that is sign switch is on. However I have switch exposed and I never found it "on" because of water.
ESC was already commented by Tim, I fully agree. I am not sure which manufacturer impregnates them, I can speak for Jeti, SPINS for C/L have 3 layers. and I know other Czech maker also covers them, so I hope it is standard on every "normal" (means not extremely cheap) ESCs. Covered is only PCB, so not cables and not capacitors. Anyway, it is not good to have water inside, so I seal the shrink tube with silicone on front side (at motor leads), so that the back side is open - it can breath and if some water is anyway in, it can evaporate. In case of combat I will seal both sides.
Timers (PCB, cables, switches, connectors) are usually not sealed, because they can be closed inside fuselage. So it is good practice to close them in waterproof coverage.
Battery will also not suffer from water in short time, but remember, there are aluminum leads, they will corrodate in humid condition, and that is problem for all those parts especially motor. It is not very funny to see rusty iron stator