Chemical weapons have problems like poisoning water tables, I've said it before but the Ukrainians should use flamethrower tanks, because they're amazingly effective at terrifying the enemy.
During ww2 the German army wasn't as a whole afraid of much of anything using conventional weapons(well besides bazooka Charlie and some Fins, but those were both grade A psychological warfare) but the British crocodile was incredibly effective at causing surrender , basically abman might not be afraid of being shot or bombed will generally still be terrified of being lit on fire
Just to complement your take on the flamethrower.
Many times, the crew on tanks with a flamethrower, would not ignite the fuel. They would just go up to a bunker, spray a bunch of fuel on the Germans and wait for them to surrender.
Just the fear of burning alive made everyone surrender instantly. And of course, if someone is dosed in gasoline, they are very unlikely to shoot their guns and spark a fire.
On the other hand, on every side, anytime the crew of a flamethrower was caught, they were treated miserably. Everyone considered the flamethrower to be a particularly inhumane weapon, so they took revenge on these crews.
Also, a lot of people refused to use a flamethrower for moral reasons.
It might be strange that in a war with so much death and injury, that people would have such hatred for the flamethrower. But being burnt alive is one of the worst deaths. It is also a very grim way to kill another human.
About chemical weapons, their effect is even worse. To this day there are still huge regions in France where it is forbidden to dig or cultivate the ground, on the grounds where chemical weapons where used during WW1.
Even after a century the soil is still contaminated and no one wants to risk it. And it will probably remain so for another century.
During WW2 both sides had huge stocks of chemical weapons, but fortunately cooler heads prevailed and no one used them in battle. But all sides were ready to use them, if the other side used. It was a bit of a cold war effect, before the col war and nuclear weapons.
Unfortunately, they were used in the gas chambers during the holocaust.
I really hope Russia doesn't use chemical weapons o Ukraine, as this would mean a harsh cost for future generations.