I find it it hilarious that the choice of argument about balance is Idoneth vs FEC. Like you do realize that Idoneth are a top tier army and FEC are no where near their power level, right? You can’t just flatly compare one specific ability vs another specific ability, you’ve got to include the whole package as Idoneth seem to be getting by just fine with that command ability the way it is now, whereas FEC definitely need something to boost them up.
The way things are now that spell alone would not be enough to bring FEC onto the top tables, but it’s a start depending on what else they get. Remember what was said before, FEC have a very limited selection of units and very little variety between those few units. If the army is a one trick pony, then it needs to be able to do that trick better than most other armies, so you will find situations where yes this one spell/command ability is just flat out better than the one in this other army, but there will be other factors in play to help balance it.