Exactly what are the risks of an out-of-date airbag?

Is it (a) a risk of failure to demploy in an accident (the functional equivalent of getting a nice wooden wheel without an airbag) or (b) the risk of erratic untriggered deployment when there is no accident (which might well result in one)?

If (b), how likely is such a failure? I think I could probably live with (a).