All ungor variations are expendable.
All goblins on foot are expendable.
Only the peasant mob is expendable.
Notice something? I don't see anything that justifies this imbalance. Not even peasant archers are expendable fer crissakes!
The TT rule for Bretonnian Knights was that they ignore the morale checks from all fleeing infantry units, so all infantry, except for maybe Battle Pilgrims and Foot Squires should get the expendable trait.