I tried to play an Immortal Empires campaign for Daemons of Chaos a couple of times on normal difficulty.
Around the 50 turn mark, the Warhost of the Apocalypse under Archaon the Everchosen will declare war on you. Archaon's army will be pretty powerful, but it's a winnable battle. The problem is is that Warhost of the Apocalypse will usually have more than one army and they will have two Norsca vassal factions that will also declare war on you and attack you, Kul and Tong. Kul and Tong will make a straight bee line for you and attack you once Warhost of the Apocalypse declares war on you, oftentimes with more than one army each. I've lost the campaign twice now. The problem is that too many armies from three different factions will attack you too early in the game for you to be able to repel them. I feel like their aggression towards you should be scaled down, at least for the two Norsca vassal factions, Kul and Tong.
I've won several campaigns on hard difficulty playing as other races in Total War Warhammer 2 and 3, so I really don't think my own skill level is the problem. The fact that I've lost this campaign twice now on just normal difficulty should be pretty telling that the Daemons of Chaos campaign in Immortal Empires is too hard.
(The rest of you can comment and tell me that I just suck, I don't care.)