RE: Flashbang + Elemental Storm, wouldn't Canister Bomb + its transmuter (with ES) do the same, but faster?
No because you can do like 5 flashbangs in one second with high cast speed. And just do it over and over. So it completely confuses huge mobs in perpetuity and spawns more storms. Canister itself is a great AOE attack, but you are better served putting storm on flashbang and using canister as extra damage. And flashbang's fumble works on heroes/bosses unlike stuns.
Similarly I believe thermite mine can actually proc one elemental storm per mine with independent recharge and can do ALOT of damage if you get stuff to stand on all 6 at once. However in practice flashbang seems to work a bit better especially if things are spread out (like cultists) as you can spread it around as you machine gun it. If you want to proc Devotion stuff you want to use thing you can cast alot or that have a high tick rate (grasping vines, aether ray etc). In theory with the reahrage/duration you can overlap 3 storms, Canister won't be able to do that and flashbang is radically superior AoE anyway although having the mobs wander can be annoying. Also keep in mind a maxed out flashbang is over -200 DA. It is bar none the best DA debuff in the game. That is a lot of crit and make procing Ulzuin's Pyroclasm very frequent even with my crappy 1250 OA at in level 67 Elite. By putting Storm onto flashbang you can start each fight by machining 5 flashbangs and get 2-3 storms, confuse almost everything, therefore taking little damage, and have increased your OA, effetively, by a lot for whatever your heavy hitter is.