I'm also in favour of keeping water out as far as humanly possible (from the radio that is). I'm sure sprays like these will help to deal with whatever water still gets in, but you have to consider the places water will go (everywhere !), that the spray won't. Whilst you can probably deal fairly effectively with the underside of a P.C.B, you have to consider the component side as well, the spaces under low fitting components, the fact that the portion of the component lead facing the underside probably won't get sprayed etc. Some components (eg IF transformers), have screening cans, so water can get inside. In the case of servos, you also have electromechanical components to consider. It only takes a single drop of water to stop a receiver or a servo (or a failsafe) from working - depending where it goes. Again depending where it goes, it may be temporary, or permanent.
I, personally, favour using a separate failsafe (preferably fully sealed, and preferably directly fed through a 'Y' cable from the battery/switch harness), over a Rx with built in failsafe functionality (if it gets wet, the failsafe functionality will quite likely disappear with the receiver functionality).
Just my thoughts, opinions, and preferences...
Ian