The LMB kills anything which is not armored on Cataclysm. On difficulties below it is even more effective. The RMB kills anything which is not CW or boss on Cataclysm. On difficulties below it is less effective.
But starting on Legend and even more so on Cataclysm it is devastating due to no cleave limit and the high damage. There are only two enemies which you would have to keep the whole 10 seconds in the fire cone for it to kill. The rest is dying faster like for example all horde enemies. Just place it on ledge drops or spawn points or close to the melee front line and it is working like a lava pool. And you can easily place 4-5 fire cones at the same time due to the low cost. There is a Stormvermin patrol? Place two fire cones, stagger them twice and they are all dead. The higher the density the more effective the RMB is. The lower the difficulty the more effective the LMB is. The weapon is overpowered.
Granted there is not much which would have to be changed as it has a lot of intrinsic balance qualities with regards to talents (even if completely unintuitive). Personally, I would start with 20 % less damage but 25 % more burn time. Would be nearly the same overall damage but need a tiny bit more effort to keep enemies in the affected area. Not sure if that would be enough.