It burns and produces a toxic gas, which is kinda borderline under the CWC.

Also it's not meant to be used against human targets. In the military it's intent is as use to create cover via smoke. The smoke launchers on APC's and tanks are WP to create large areas of blocking smoke instantly. as opposed to what smoke grenades do which just kinda spill out color for a while.

Also, If it get's on you, it doesn't extinguish with water. You need to not only cool it but smother it because it will react and start burning when exposed to air.