[ALittleLight]

This makes me think of an observation from (I think) the web-serial Worm. Being a super hero who shoots fire is very tough. Either your opponent is someone who is vulnerable to fire, in which case your power would burn them to death (which is not very heroic) or your opponent is not vulnerable to fire, in which case your powers are useless.

Similar thinking seems to apply here. Either your target is a modern, mobile, equipped army of professionals who will take minimal damage from chemical weapons, or they are a vulnerable population of civilians who will be massacred horribly if you use them.

[derefr]

War isn't like superhero/kaiju duels, though. Duels are generally about the opponent themselves — you can only win by disabling them. But a large part of war is about the logistics of moving soldiers and supplies around and through places; and so it can be critical (even war-winning) to deny your enemy the use of territory.

A "superhero that shoots fire" (i.e. a guy with a flamethrower) — or a rolling cloud of nerve gas — isn't there to actually harm anybody; it's there to be a thing your enemy sees and stays well clear of. It's an https://en.wikipedia.org/wiki/Area_denial_weapon. The threat of it prevents your opponent from coming within range of it; and thereby locks them out of the area it threatens.

And, as it happens, a flamethrower is a much safer area-denial weapon than a cloud of nerve gas — as, whenever the enemy isn't around, you can immediately shut the flamethrower off, and use the area yourself.