I've been playing a Festus campaign and I upgraded a Mortal Lord into a Daemon Prince of Nurgle. I was shocked to find out how much better the Daemon Prince seems to be over an Exalted Great Unclean One. Here is what the Daemon Prince of Nurgle provides: He has the same Mortis Engine effect as the Exalted Great Unclean One; he can fly at 80 speed; he has access to both lore Passives of Lore of Nurgle and the Lore of Death; he has access to the Herald Locus ability that buffs Weapon Strength in an AoE; he has a passive AoE Weapon Strength debuff to enemies; he has increased casualty replenishment for his army.
The only areas where he seems worst (though more like a sidegrade) is that he has a mixed lore of magic so he doesn't have access to Stream of Corruption, Blight Boil, or Fate of Bjuna (he does have access to Spirit Leach, Fleshy Abundance and Black Sun though). He also has less HP than the Great Unclean One.
I haven't played with the other Gods' Daemon Princes so I don't know how they compare, but if they are in a similar situation to the Nurgle one, then they are also probably much better than their Gods' Exalted Greater Daemons. I do Know that the Daemon Prince of Tzeentch has access to the AoE passive Barrier regen as well as an entire close combat Yellow skill line, which the Exalted Lord of Change lacks.
How should this be improved? The main reason for this disparity is that the Daemon Princes have access to 6 unique skills (3 of them they share with other marked characters) in addition to the standard skill lines, while the Greater Daemons have access to almost none. I would like to suggest that the Exalted Greater Daemons will get a few extra unique skills that will make them truly stand out.