Truth is that Boom is pretty stronger than Dk Garrosh and Hagatha is stronger than Dk thrall. But by no means you can compare Boom or Hagatha with Dks like warlock, mage , hunter, priest.
As for Suljin, well yes he swings the game but you got to create a deck full of spells in order to work. Why you consider him better than Shudderwock or Dk guldan or N’zoth
Shuderwock is pretty strong too but I still think suljin is better because Hunter spells are that good. There are many ocations you don’t wanna shuderwock on curve cause it doesn’t do enough yet but that’s rare for Suljin.
Gildan is amazing with doomguards and big demons but I still think suljin is more flexible when it comes to deck building and does a lot more than giving you a board.