How to achieve "fall off the bone" in a leg of lamb using sous vide?
Ribs and legs are "done" when they are 145°F internal temp, but they may still be tough. If you take them up to 190 to 203°F, the collagens and fats melt at this temp and make the meat more tender and juicy.

Alot of people prefer caramelized and easy to eat meat. Altough I would not call this sous vide.

A longer cooking time would not melt the fats and collagens, but tenderize the meat more.