I haven't played as Otomo or Uesegi, Hattori were hard but I didn't find the other ones that bad. Also had the same problem with FotS, once you get the best artillery the game becomes cake, and I usually just stop playing because it gets so boring. Anyway while some of the factions in Shogun are hard to play as, still got nothing on Alexander or the Kingdoms campaigns, now those were Hard!
The Otomo get special benefits for being friends of the Portuguese like the ability to lease land to them and hire Tercos as well as cheaper Matchlock Ashigaru. They're Christian though so everyone hates them, it's tough. The Uesegi I liked because I prefer Warrior Monks (their specialty) to Samurai but the Takeda stomped me pretty early on. I never had much trouble with the Kingdoms campaigns but I'll agree with Alexander, it's probably the single most challenging campaign CA has ever produced.
I'd like to try another Fujiwara RotS campaign, I guess the best thing to do in RotS is immediately kill your relatives so they can't ruin everything a decade down the line?