The inferno units are not among the more powerful of Heroes III's creatures. The imp is a terrible first-level creature, but you should keep a small stack of upgraded ones in your army to steal spell points from enemy spellcasters during combat. This is a useful special ability. The gog is a fair ranged unit, while the upgraded magog is a dubious warrior because its area of effect damage can harm your own troops. Compounding this problem is that the magog is slower than most of the other inferno troops, so by the time the magog can fire, his cohorts are already surrounding the enemy and thus in the line of fire. The hell hounds are decent troops, but cerberi, with their ability to strike three opponents at once (without fear of enemy retaliation) are great mid level troops. The demon and pit fiends are middle of the road creatures, tough and relatively strong, but not in the same league as the crusader or minotaur. The efreet has good speed, attack, and damage, and it is the lone flying unit. The devil is a good seventh-level troop, but the upgraded arch devil is much better, able to teleport anywhere on the tactical map and gaining a huge speed enhancement. Physically, though, it is weaker than the behemoth, dragons, archangel, and titan. Fortunately for inferno heroes, the enemy cannot retaliate against devil attacks.
Upgrade your hell hounds as soon as you can. The other creature upgrades do improve their respective creatures, but not very significantly, so it's not a priority. The horned demon, for example, only gains +5 hit points and 1 speed. You might want to consider upgrading your imps to familiars so you can get their magic channeling special ability and upgrading efreet to gain their +4 speed jump and the fire shield defense. As for the other upgrades, only do so when you have the luxury.
Prolific low-level creatures
Strong in magic
Can teleport between inferno towns
Fast low- and high-level creatures
Only one ranged unit
No low-level flying units
Some upgrade buildings require precious resources
Poor upgrades to creatures
Thanks to The Nether Gods team for the contents of this page!