Nastiest situation is one uc leg stuck up and the other one down. Most jets have an emergency lowering system of one sort or another. Often this is a compressed air bottle which feeds air to the down side of the jacks when a diaphragm is ruptured electrically by the emergency down button. So far so good, but if one leg has failed to come down and pressing the emergency button doesn't lower it then the other one is stuck down and cannot be raised. This rules out a landing on an even keel. Hence the decision to press emergency down may not be straightforward.
With very swept wing jets the advice is to eject if one leg is down and the other up. That said, a Hawk has been successfully landed on one main leg without too much damage. The wing was held off the ground with aileron for a surprisingly long time.