Have looked at this quite a few times in flight test of new (light aircraft) prototypes. The reason we've done it has been just this - the need to satisfy ourselves that the handling will be okay if the flap actuator / linkage on one side fails.
Being very generalistic here, the answer is that there is always a degree of handling asymmetry, requiring the pilot to maintain (usually) some aileron to keep the aircraft in balance. However, the force has generally been a few pounds, and the workload whilst it has increased, has not done so exceptionally.
So, should this happen, yes you'll be sweating a bit, but I think that the chances that you won't be able to control the aircraft should be pretty small.
I should say that one of the underlying assumptions of the flight testing we've done has always been that you'd not try and land with asymmetric flaps, but balance them out and land with symmetric flaps. For that reason, we've not taken these asymmetric tests to the stall or landing - the testing risks weren't considered justified.
I'm obviously not giving any detailed information about specific aeroplanes; that's confidential data that I can't share publically, you'd have to ask the individual aircraft manufacturers. But, I've yet to see any type where an average PPL would be unlikely to cope.
G