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I wouldn't call Inferno a weak faction either. And I can explain why.
First, a few words about the units:
Imps are nasty little creatures that steal mana. If there are a decent number of these little guys in the army, the enemy Hero risks running out of mana on their very first turn. They can also harass weak enemy units.
Demons are quite resilient but slow creatures. IMHO, they are less useful than Imps.
Cerberuses are similar to Death Knights in the High Elves. Fast units that attack several nearby targets, and most importantly, they cannot be retaliated against.
Succubi - I don't like playing against them, to be honest. They need to be killed either with one volley or with melee units, as their retaliatory attack can do a lot of damage to your ranged units. They are the only ranged unit in the Demons, and not the weakest ranged unit in the game. And I almost forgot about the number of targets they can hit - from 1 to 4 (although the damage weakens, it's still there). They don't have many shots, though...
Nightmares are slightly weaker than Unicorns, but faster and with a nasty aura that lowers the morale of nearby enemy units + fear - like blindness in Unique units, if it affects an Ent... save your ranged units.
Cave Lords - to be honest, I've become a little disappointed with them over time. My first encounters with them didn't make me very happy (as with Druids). If they had a stronger meteor shower and more mana... but as it is, 1 meteor or 2 balls to choose from. You can basically use them to cast a destructive beam on enemy tanks. As a melee unit, they are not the strongest either - decent damage, but speed and initiative... They are more of a minor nuisance due to the Meteor and balls. You can't really fight with them.
Archdevils are medium-strength level 7 units. Decent attack. Teleport. They can summon Cave Lords. But overall, nothing special.
Now about the Gate: Personally, when Demons attacked my Castle, it was much more of a hassle than with other factions. Because first, Imps, Demons, and Cerberuses open the gate right in front of the ranged units. And if the Hero has this skill developed... there can be problems.
As a result of the above, this is my IMHO about Inferno:
It's a purely offensive faction, as there are many melee units - and very fast ones (Nightmares, Cerberuses, Imps, and Archdevils). The Gate is also not designed for defense. Heroes have one of the main parameters as Attack (defense is almost not developed). They are one of the best at sieging castles. So if you play skillfully, the faction is quite capable, a solid mid-tier.
P.S. I myself am a Sylvani, but I tried to be objective, so don't stomp on me
