I wouldn't say that demons are the weakest, and it's not about a super ability. Besides the Castle, the other factions have super abilities that are just as good.
And what's wrong with their army? Let's compare:
IMP – 1st or 2nd place.
Demons – 5th place
CERBERUS – 2nd or 4th place
DEMONESS – 1st place
HELL HOUND – 2nd or 4th place
CAVE LORD – (an interesting question)
ARCHDEVIL – (2nd or 4th place)

I'm sure many will disagree with your ranking. Personally, I completely disagree with the Demoness and Archdevil. The Demoness is highly dependent on the situation and can lose to any level 4 unit (although she can also win, but I would put her in 4th-5th place), and the Archdevil only wins against the Ghost Dragon (although in a battle of 2 against 3, the victory will be with the dragons due to the curse), because level 7 is very balanced.
In my opinion, in the Inferno game, the main thing is to accumulate Imps and devour (preferably completely) the enemy's mana, and then Dark Magic, good initiative, and attack will do the rest. If this fails, there may be problems. But the Castle is still an interesting faction.

With the introduction of the Dwarves, things have gotten even worse! Their ultimate ability is not just powerful, it's TERRIBLY powerful.

It's not more powerful than the Elf or Necropolis, but that's not important, because the ultimate abilities themselves are found in nature even less often than Bengal tigers. It's strange that they haven't been added to the Red Book yet.