Today’s weapons are produce some pretty terrible ways to die but are not that easy to disperse efficiently and tend to have a short period of effectiveness.
Sunlight will kill most of the agents and rain washes it away. They will kill most people during the initial attack and serve as a tool for area denial.
Using such weapons today is the death sentence for any nation or army in that the first use of such weapons will bring a massive and more lethal response from a military such as ours. The far more deadlier use of these weapons would be as my late grandfather used to say would be in a city. As he used to say a canister no bigger than a thermos bottle introduced into the ventilation system of a large office building would wreak havoc.