Spellcasting is slow if we compare with the Multiattack option the monk has. At level 5, a monk can do up to three attacks without spending ki, wich is pretty great. A Sweeping Cinder Strike, on the other hand, cost 1 full action: no bonus action from Martial Arts can be made if no Melee attack is intended, nor a Multiattack option, because it's not considered an Attack. Here, FOTFS is a bit better, as you can redirect attacks and you are not limited to the cone. Not a lot, though.
I have been Houseruling some powers for elemental monks, but I feel they are a bit unbalanced yet. I would appreciate some help to balance it. It's now in Spanish, but I think in a few days I could translate it.