I'd say false. Effective, only in small spaces like indoors (it would becoem too diluted outdoors unless HUGE amounts were put into the area) or condensed. Reliable, same issue as effectiveness, though maybe some batches were weaker than others. Easy to control- about as easy to control as smoke with bare hands. Once poisonous gas was released into an area, it went everywhere air could go. Unless you had the means to directly control it from its start point to an end point, like in Hitler's gas chambers, poisonous gas was a very volatile weapon.
Not to mention, it was fully pointless if the intended victims had gas masks.
Sorry to get a bit dark there, but I hope it helps!
