Chemical weapons are a problem because they usually do not kill. It takes a LOT of chemicals and the right environment to kill. But they do tear up lungs and eyes and nervous systems. So the casualties may be able to move themselves but they cannot pick up their old lives again.
I read somewhere that the overall cost of chemical weapons and conventional weapons per casualty work out to be the same. Chemical weapons on a per-round basis may be more effective (when used to the greatest tactical advantage, and a wind shift can affect that quickly), but the resource costs (additional training, transportation, storage, and protective costs) associated with chemical weapons neutralizes the advantage overall. That military foes will almost certainly already be trained and equipped to deal with a chemical attack further reduces their effectiveness.
Their only effective use remains as a terror weapon, or at best an area denial weapon on retreat to slow the enemy (it's much harder to move as quickly when you're wearing a mask and possibly other gear).