RE: J3 cub, crash on take off nearly every time.
No, wind speed makes no difference whatsoever as far as the plane is concerned. If your Cub was getting slow enough to stall it's because you were slowing it down involuntarily in order to keep it at a familiar speed relative to you: groundspeed. Now, there is a danger in flying slowly in GUSTY conditions, and that is that as gust intensity increases, so must airspeed, for safety. An example would be flying stable at 20kts and suddenly getting nailed with a 5kt gust in the direction the plane is flying: airspeed drops to 15kts and then rises back to 20 as the plane seeks its original (stable) 20kt condition where all forces are balanced.
It definitely looks misleading from the ground, when your plane is hauling booty downwind and yet you have very little elevator control and are near a stall. But it's because you are deliberately throttling back and pulling the elevator back to make the plane look "normal" from the ground. The thing with RC models is we have no speed reference other than groundspeed, so we must consciously consider what the wind is doing and factor that in. Consider this example: a plane is flying normally in a stationary mass of air. Now imagine the ground is zooming by beneath it at 20kts. How does that affect the plane? It doesn't.