My experience is... Inferno sucks. BUT I feel that this is more related to my mistakes or unforeseen circumstances (randomness) on the maps than to the balance.
Bad heroes? Well, starting heroes rarely become the main ones.
Weak overall damage and HP? But the speed is one of the highest + retaliatory strike for two units at once (for level 7, this is really cool).
With the right tactics, we will most likely force the enemy into melee combat from the very first turn of the battle, and then Inferno will hardly lose to anyone.
Anyway, the final army is either just ifrits with Armageddon, or devils, ifrits, hordes of demons, and a decent number of cerberuses, on which the enemy will be forced to spend time.