Think of them as simply ailerons able to droop (into flaps) So what’s the advantage? They are part of the lifting surface of the wing (note the airfoil shape). As the wing reaches the critical angle of attack (stall) and turbulence (separation) is generated on the top and to the rear of the wing. The Kitfox detached Flaperons (Junkers style) are not only hanging below this airflow disruption, but they have not reached there critical angle of attack. The airflow is being accelerated over the top surface of the Flaperon – from the downwash of the underside of the wing being funneled through the narrow gap. Therefore you have aileron authority deep into a stall. (reflect on a Cessna trainer – loss of ailerons – only the use of rudder lifts a dropped wing in the stall).
So why hasn’t everybody adopted this design? Many have, they are in use today on many aircraft but “in trail” The detached flaperon has been used in the past for low, slow, highly maneuverable airplanes. eg. Storch and Junkers.