The Dwarves also have a rather difficult start, and so what?
You can also have a good start with Grok. The ability to get "surprise attack" after just a couple of levels allows you to break through many melee units and almost all ranged units with just Cerberuses. You just shouldn't attack mages, druids, and dwarven priests unless you can kill them all in one turn.
Inferno is considered weak only because many people are simply too lazy to bother with demon units, which require constant attention, otherwise the losses will be enormous. But with the right skills, demons turn into a formidable force that you have to reckon with.
Only Grok has a useless skill at the start. With a tiny amount of magic, the Chaos Demon is almost useless. Perhaps the only justification for Chaos in the demon's arsenal is withering flame/fire rage/cold steel. But it's not always worth spending 5-6 levels of development just for them; there are better skills.
Everyone complains about the cost of Tier 7 creatures, and the Devils are no exception.