Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.
It's exactly the same.
Once the airplane is in relatively steady state (straight & level, upwind or downwind), few pilots have problems in this context. The cited problems usually occur while in the turn to downwind, with the pilot trying to force the airplane to fly a specific track over the ground instead of a simple climbing, constant-bank, constant- (though slightly increased) airspeed turn in the air. It is the reduced stall margin in the turn (if the airspeed is not increased), combined with the gust factor, that will get some into trouble.