I don't understand anything. In the early game, losses are inevitable in battles against archers or mages (so everyone is on equal footing), but summoning from the underworld is very helpful against melee-oriented neutrals (except for high-initiative ones, like dancers, who can reach the summoning point before reinforcements arrive), especially against tough ones. So, what's so difficult about it?
Of course, you need to know the possibilities of initiative to calculate the moves between the moves of the neutrals and skillfully deploy summoned troops. (And they could have made a low-level upgrade that shoots, or at least jumping demons :))

Well, firstly, all the low-level creatures of the Demons need early upgrades because they become significantly stronger, especially the demons and dogs. This makes city development more difficult. Secondly, the lack of low-level ranged units also makes their life harder. (By the time demonesses are built and their numbers become large enough, a lot of time will have passed.) Not many creatures are summoned, but they can be really useful; for example, I always use them to retaliate. But the autobattle doesn't know how to do that. The troops themselves aren't very durable for the Demons. The new demons (level 2) aren't as cool as they say. Their defense is minimal, and they die quickly. The minimum and maximum damage differ by as much as 4 times, which makes each attack a kind of lottery: you might get lucky, or you might not. And this complicates the use of their ability, because if they kill few creatures, they will take a lot of damage (the counterattack damage is doubled), and there is not much defense for either the creatures or the hero. The saving grace is the dogs, which should be built in the first week and upgraded as soon as possible. Against slow neutrals, you can do the following: deploy the dogs, they run up, attack, and run back, if the initiative allows. And at the same time, attack with the hero and possibly shoot with a ballista. Cerberuses are not furies or fairies, of course, but they can sometimes be helpful.