I cant comment on N'Kari but there is quite a bit of difference in difficulty between all the factions/Lords. Some campaigns like Grimgor or Tyrion are pretty straightforward for example (even on Legendary).

N'kari has a rough start considering you're surrounded by high elves. It's all about stabilizing your starting position and building a snowball of cultists and gifts of Slaanesh until you are rolling in so many devotees you don't know what to do with them all. Easier said than done though. I fucking hate siege battles as him though so I try to lure enemies into land battles with op as fuck ambush stance. If I remember right Forsaken are the way to go early against high elves, they have the armor to close the gap and butcher spearmen and archers.

You can always just abandon Ulthuan and return once you are op as fuck. Bonus points for proliferating a cult in Cathay, summoning a disciple army there, and relocating just to seduce and vassalize Miao Ying. Or Imrik if that's your thing.