The argument I've seen against their usefulness is that they only work against static militaries that don't have NBC training, and a modern military can already defeat threats like that without paying the political cost of using them.

https://acoup.blog/2020/03/20/collections-why-dont-we-use-ch...

> Chemical weapons are incredibly useful

They're not. They're effective against an unprepared adversary. Any real military will have anti-chemical-weapons tactics and protection.

So this happens: you douse a building with a gas, and then walk inside and get shot by defenders in gas masks. And by the way, you also will have to wear a gas mask yourself.