When deflected downwards they help let the plane fly slower down to around 20 degrees angle. At this point the adverse yaw they cause is managable if you use some rudder with your aileron inputs. Beyond 20 degrees things get real nasty in a hurry.

When delfected up they still have a yawing effect but now it works FOR you rather than against you so you can use more up "spoileron" angle than for the "flaperon" angle. But it'll make your plane descend like a gut shot duck from the drag and reduction in maximum lift possible and the touchdown speed may actually be higher.