One has to be cautious here, because no two aeroplanes are the same, and I obviously only know about the certification programmes that I've been involved with - which is quite a few, but clearly tiny compared to the number of aircraft types around.
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.
As for aileron power, yes I'd be very surprised if there was insufficient aileron power to counteract the effects of asymmetric flaps - just look at it this way, you've got two ailerons (fighting one flap), and they are twice as far (roughly) from the centre of the aircraft as the flaps. So, being incredibly simplistic about this, I'd expect to get about 4 times the roll-power out of both ailerons as I would out of a single asymmetric flap.
G
N.B. So far as certification requirements are concerned, "it depends" there are a lot of different sets of rules depending upon aircraft size and intended use, and a lot of caveats to do with how the flap system is constructed. Add into that different professional opinions amongst the like of I and my colleagues about how each safety requirement should be met - and you get "it depends"! What IO540 has suggested however I'd certainly not regard as surprising or unreasonable if I found it in a test programme.