[Kobal2]

Devil hordes work nicely with Darkness (which they're immune to), fire evocations (same) or spells that cause fatigue through heat (again, immune). Since they're demons, they also get their own Blood buffs on top of whatever regular buffs you'd wish to cast on your troops. Their high MR and high HP means they're not too vulnerable to priest spam.

Finally, their heat aura makes non-elite or heavily armoured melee troops very useless, very quickly. 10-15 devils will rip right through almost any amount of PD. If you've got flying commanders to lead them (preferably with Astral so they don't get Mind Hunted before they take off), they're excellent, upkeep-free raiders.

And as thejeff says, if you get to cranking the contracts early and in force, you'll get ridiculous amounts of them in no time. See Baalz' MA Abysia guide for details.