Well, Demons have very high initiative, which, combined with their very high attack, can cause a lot of trouble. Also, for long maps, there is an excellent hero – Nimue, who can summon twice as many creatures as the initial limit. And, of course, there is a great trick with summoning reinforcements of Cave Lords from those summoned by Archdevils.
For a decisive battle, the Demons' racial ability is one of the best.

The start is difficult only at the beginning. Once the Succubus (Temptress) is built, everything becomes much easier. And even so, Cerberi + jumping Demons, taking into account the summoned creatures (which absorb retaliation!), work quite well, destroying neutrals. In some versions, there are even funny situations where reinforcements are summoned, and the AI doesn't want to spend a turn destroying the summoned creatures from the gates, resulting in a long dance around the summoned creatures, while the hero calmly shoots down the neutrals.

Another thing I like about Demons is that the hero doesn't have very high magic skills, and spellcasting rarely appears, so there's no need to overthink things – Demon heroes are simply made for general Dark Magic spells, and at the same time, the hero doesn't move too far on the initiative scale, which justifies the large amount of mana the hero has. Thus, the turns of a Demon hero are generally quite effective.