I don't think mustard gas is that bad. You can't stand in it, but it there are multiple ways to counter it: sipping healing, regeneration, gas immunity. And to prevent it, you still have searching (with the ring, manually it's way too tedious) and levitation.
Gas immunity is even rarer than levitation though, you never get it when it actually will be useful, which is GC 1 - . After that you get enough END to deal with it. Regen is very weak too, so sipping is pretty much the only counter anyone will be using.