A flaperon is a combined flap/aileron. They are setup primarily with computer mixing but can also be done mechanically. It requires 2 servos (one for each surface). They are used on gliders and some flying wings (I think). Generally speaking they aren't used in sport models. If I had to venture a guess I'd say that they probably greatly increase you chance of stalling one side of the wing and crashing. Consider this scenario:

You're setting up for landing, and you deploy the flaperons. As you turn onto final the aileron at the inside of the turn (up) returns the airfoil essentially to it's original shape. Bang! The wing stalls and you have a bunch of sticks. Now I now that someone will say flaperons or not this could happen, and they're right. But the flaperons will make it much much worse and more likely to happen.

Conversely, if you consider the down flaperon, at some angle it stops providing lift and just turns into a spoiler. That'll give you at a minimum a lot of adverse yaw and at a maximum control reversal.