Infernos are only overpowered if you're a crappy attacker.
Then why would you judge farming with healing spell incredibly inefficient?
Just because you use whatever method you use and it makes other methods inefficient?
Not that I use healing spell to farm.
There are many ways to farm. They fit in various playstyle and trophy range. And it depends on your TH level too.
TH 10.
Because archers get one hit by mortars and high level wizard tower giving heals no effect. Barbarians get pummeled by defences faster than they can heal which gives heal no effect. Giants are the only troop that benefits from heals because they have enough hp to take the initial damage and heal while the defences are "cooling down." Goblins suffer the same fate from Wiz towers. Those are most of the farming troops. If you farm with Giants and Wizards, in a DE farming army, then yes that is an efficient way to farm. However, I was referring to Gold and Elixir farming.
I think the Inferno Tower is good as it is. Not weak, nor OP. Multiple target just means using cheaper troops to tank the damage.