"Weak demons"??

Furies don't always eliminate everyone at once, but they are often the primary target for enemies. Even single spellcasters and archers won't reach them unscathed on the first try without tactics and boots of speed. And assassins won't be able to eliminate everyone.
Demons are unlikely to be significantly worse. They might fire once, but who won't they manage to fire at?
For example, compare Elves and Demons in a battle against archers, especially when both sides have other units in their army, particularly if there are 2 or 3 groups of these archers. The archers will fire once at the Demons, but then the Demons will block the path and do their job along with the Cursed Sign.
The Elves will definitely take at least one hit before they reach the archers, and then the Blade Masters will retaliate, if they don't finish them off immediately. The Fairies might strike without taking damage, but if they don't finish the job, they'll get hit again, just like the Blade Masters. But the archers won't attack the Demons again. As a result, the Elves are unlikely to suffer fewer losses, and may even suffer more.
If we consider the High Elves, the Furies also won't reach the archers immediately and will take a few hits, because the assassins will only poison one group, and while they're dealing with everyone, the Furies will get hit again. You can try using only Hunters, but that's 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 the Elves, but it doesn't help the High Elves much. The Demons' dogs can reach the enemy quickly, but only with additional speed, otherwise it's the same as before.
The Undead also suffer a lot of losses, but you can resurrect them, although you won't have enough mana for everyone at the beginning.
For humans, the archers on the second level help, but there will still be losses.
The same situation applies to the Orcs.
The Dwarves can reduce losses by keeping their first-tier units, but they will take too long to deal with the enemy, and if they deploy Javelin Throwers, they will suffer losses. The Dwarves suffer heavy losses at the beginning, and only later do they become overpowered.
Magic is the only thing that saves the Mages; they have enough mana at the beginning, so they can minimize losses.
So, it's not clear why the Demons would suffer more losses. But this only applies to archers. And if we consider melee units, it's even easier for them, especially against larger melee units.