Nonetheless, I would not expect a sudden loss of control, I'd expect asymmetric flap selection to cause a marked but not massive roll rate towards the wing with lower flap deflection, and probably some yaw away from it as well. I would not expect the control forces needed to hold the aircraft level to be massive, but I wouldn't want to be holding them during a landing if it could be avoided, not so much because of the workload, as because of the uncertainty over the stalling characteristics.
I had an asymmetric flap retraction on a PA30 with one flap remaining at about 15 degrees while the other retracted. The control forces were very much as Genghis suggests -- marked but not so strong as to make the aircraft uncontrollable. In my case the problem was soon addressed by re-extending the flap.