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This question interested me - I did a bit more investigating. I found plenty of anecdotal evidence that many pilots have survived a full asymmetrical flap deployment in many kinds of light aircraft. This makes sense - you have two ailerons counteracting one flap, and the center of pressure of each aileron is farther outboard by a factor of more than 2 (x3, if the flaps and ailerons are of equal span.) Every crash I read about came from a problem that occurred close to the ground. The only crash of a large airplane I found was AA 191 -the DC-10 at O'Hare that lost an engine. That flight was under control until it dropped below the slat-retracted stall speed.
A few people on the internet thought FAR 23 required that the airplane be controllable in an asymmetric condition - I could not find that explicitly, but I did find the FAR that says the wing must handle the load (and not snap in half.) FAR 23.701 implies that a possible asymmetric flap failure that results in an uncontrollable condition will pass certification if the failure is "extremely improbable."
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