I think the main "problem" with mustard gas is that it's a new damage type. Think about the other types of damage in the game:
* combat damage - healing liquid
* exhaustion - water
* fire - douse with water
* leprosy - antidote
* poison - antidote
* lost limb - troll blood
* damaged artificial limb - repair
All the other types can be fixed and healed instantly. Mustard gas introduces the first type of damage that lingers even after it has initially been healed.
Yes, intuitively, all of us get out of the gas as fast as possible. The problem was that I would always drink the full bottle of healing liquid immediately. Which is actually a bad thing, because I don't need the full bottle to heal the present damage so it's mostly wasted, and now I have nothing to combat the lingering damage.
Now that I know better, if I keep tasting or distribute the bottle into vials and drink one at a time, it's pretty easy to survive.
Older players probably struggle with this more because taste and vials are new features that we're not used to.
I don't think it's any worse than the instant kills of land mines. Or fire damage. We all learned from experience how important it is to find fire resistance. And now I know to carry several vials of water so that I can douse all of my equipment. Similarly, now that I know how to fight mustard gas, it's pretty easy to deal with.
However I do think we should add a new type of liquid (or possibly an ointment or salve) which instantly heals mustard gas. That will make it easier for new players to deal with, since learning how to deal with the lingering effect can take a lot of frustrating deaths