As an experiment I set up a basic two meter flat wing glider with full span quite wide flaperons. And yes, using small amounts of flaperon to alter the camber worked very well for both slowing it down for thermalling and slightly reflexing it for speed runs back upwind.

But bad things happened when I tried to use them as actual flaps for slowing down. The adverse yaw effect took over from the proper aileron direction. At one point around 30 degrees deflection there was NO aileron function and as the angle was increased the adverse yaw was so strong that the aileron function was actually slightly reversed. This is why coupling the flaps and ailerons for small angular changes is a good thing but once the flap angle is increased past that point you want the mix to reverse and move the ailerons back up to neutral with increased flap angle.

Also the usual really narrow strip ailerons that are only around 5% chord don't work worth a hoot. The flaperons need to be at least 15'ish% wide and 20'ish% is primo.