"Weak demons"??

I still disagree about the Inferno. You can advance against even large armies without losses. At the very beginning, Furies take out all the infantry, Stalkers take out all the archers, including mages (although the hero will need mana for this).
Demons are among the worst when it comes to fighting neutral ranged units. Before the gates open, all the archers will have time to fire once, and the Inferno creatures are, to put it mildly, not very durable (tactics and boots may not even drop later). But mages are different - you put out only Gargoyles and calmly defeat the enemies with arrows/fireballs. Or Gnomes. Improved Guards hold up well.

Furies don't always take out everyone at once, but they are the first target for enemies. It also won't be easy for them to reach single mages and archers on the first try without tactics and boots of speed. And Assassins won't be able to take out everyone.
And it's unlikely that Demons are the worst. They will fire once, but who won't they have time to fire at?
For example, compare Stronghold Elves and Demons in a battle against archers, with the former having their first-tier units in the army, especially if there are 2 or 3 groups of these archers. They will fire once at the Demons, but then the gates will block them, and they will do their job together with the Cursed Sign.
Stronghold Elves will take at least one hit before they reach, and then the Blade Masters will also retaliate, if they don't finish them off immediately. Fairies may strike without retaliation, but if they don't finish them off, they will get hit again, as will the Blade Masters. But they won't hit the Demons again. As a result, the Elves are unlikely to suffer fewer losses, and may even suffer more.
If we take Inferno, the Furies also won't reach immediately and will take several hits, because the Assassins will only poison one group, and while they deal with everyone, the Furies will get hit again. You can try using only Hunters, but only if they hide in stealth first and if the hero has enough mana, which is questionable at the beginning.
With third-tier units, it's easier for Stronghold Elves, but they won't help Inferno much. The Demons' dogs can reach immediately, but only with additional speed, otherwise it will be the same as before.
The Undead also suffer a lot of losses, but you can resurrect them, but there won't be enough mana for everyone at the beginning.
For Humans, archers on the second level save the day, but there will still be losses.
The same situation is with Orcs.
It's easier for Gnomes to reduce losses by leaving the first-tier units, but they will take too long to deal with the enemy, and if you put out Javelin Throwers, they will suffer losses. Gnomes suffer heavy losses at the beginning, and only later do they become overpowered.
Mages are only saved by magic; they have enough mana at the beginning, so they can suffer low losses.
So it's not clear at all why Demons suffer more losses. But this is only in relation to archers. And if it's against melee units, it's even easier, especially against large melee units.