Except when you're trying to get somewhere, have limited space, or are about to hit something, groundspeed is irrelevant to an airplane. Airplanes fly only in relation to the air, as far as how they work. A downwind takeoff is no more dangerous than an upwind takeoff if you have the room to accelerate to the proper airspeed and climb out at the reduced climb gradient. The wind coming from the rear will not 'blow' the tail around unless you're flying in some seriously-gusty wind...and then it's likely not wise to attempt flying at all.

With models, because we don't have real-time airspeed indication, we usually rely on the visual cue of its speed to determine when to initiate the takeoff rotation. If the model's taking off downwind, the model will be moving a lot faster than normal, so we have the illusion that it's moving fast enough to fly, even if it really isn't.

The model left the ground to early, flew out of ground effect and stalled. There was nothing wrong with the model or the equipment.

(edit for typos)