Imo the problem is that elementals units are not cost efficient.
They are all waaaaaay too expensive, making early game extremely disadvantageous since whatever you start, any other orders can beat it for a far less price, making all trades on enemy's favor. They don't need a lot of mana, but so much golds...
Start as elementals are kinda weak, so weak that it's impossible to actually be aggressive. 2 swordwrath, for slightly less golds than a earth, can easilly beat 3 earth, which make a simple sword/archer start nearly unbeatable, like said before.
Also, even later on the game, the only goods damage deal options you have involve fire elemental, that is also the only real ranged unit, and a really expensive one that ofc make the offensive Combination Units extremely expensive (850 Gold 100 Mana for an infernos, the weakest caster in the game imo, that's a lot).

All this make mass treature the only real option you have since it is the cheapest combined unit, the only one that you can make early in the game and it still scale later on.
I think there are 2 options to solve this: make the units stronger to worth their price, or make them cheaper to worth their price.

That being said, i think that elementals are extremely cool, are an extremely good concept, extremely fun, and extremely interesting to play.
But atm, even if it's my favorite empire, i just can't pick it cause i know that i have far less chance of winning while playing this.