You said it. It's pretty much the same with the other 2 campaigns. Even the Legions have a Healer (the angel in human form) that aids the main legions hero who turns to be a former elf and disconnects himself from Bethrezen - let's not forget the the Legions in Dcp2 did not have a lealer, just like the undead and the mountain clans. Every campaign is just way too easy.
What is good about Dcp3 is the storyline. It explains why Bethrezen went bad. I would as well if my bothers and sisters corrupt a wonderful world I made and I suffer my daddy's wrath because of them
Well, maybe we should make a lot of requirements for a tough game
Don't use Focus points, don't use cheesy strategies, play without capturing the resource spots, don't carry over any items from previous maps (only the clothes on the hero's back), when the game bugs and shows you the enemy city, build it up completely... Things like this.