Depends on the plane. Flappersons can be used perfectly effectively without loss of aileron effectiveness if they are full span ailerons and are used in moderation. If you have wing tip ailerons you're just going to induce tip stall by adding wash-in to the wing, and if they are deflected too far you'll have problems such as dropping full flaps but if you use them at about 15 or 20 degrees flap, you wont have any problems on most airframes.