The yaw with flaperons is the same thing, but in the opposite direction.
The surface that's down all the way can't go further down when commanding a turn in the opposite direction. The other surface, being down, goes up, lessening the drag on its side. The net result is a turn/yaw -away- from the intended direction, which is disconcerting at least, and totally disorienting at worst.
The effect with spoilerons is much better, the up surface providing the drag on the proper side.