In my walkthrough, I assign a difficulty of 3 or below if I won the mission with 0 loss - that's roughly half of the missions! Something is wrong!
I think the overall easiness of the campaign comes from the below two reasons:
(1) Healing is overpowered and ubiquitous. Life Drain, Regenerate, Healing all resurrects fallen units. If you can't cast it, you can buy a cheap scroll to cast it. While you wait for a cool down, some artifacts will allow you to cast another free spell every battle. If any player finds a mission too difficult, usually one advice will alter everything - abuse the healing abilities and Reinforcement.
(2) AI's scripted army grows too slowly (and eventually they will use up their pool and stops growing). Eventually the human player will overpower every boss, who does not get more powerful over time AT ALL. ARE YOU KIDDING ME?
If the above two remain so, we can have the good old "heroic" difficulty back, which gives us even less army and resources to fight even more enemies, but people will still find them too easy - just wait for a few more weeks and we again have the upper hand.