I would argue that mudslide is far from the biggest thing about it. The reason I would take Kurgan ook is because it can move about your models with ease with shifting earth, avalanche and chronicle.
Nephilim for instance have a hard time doing interacts if engaged. Just being within 6" of Kurgan when they activate will allow for its chronicle to trigger off regenerate, and they can push 2" before they even take any actions. Freeing, say, a young nephilim to move, interact and use fly with me.
Other uses can be to shove slow models like Killjoy or Candy really far up the board early in the game.
Edit: If you give the kurgan a little push from frightening reminder or similar, you can move another model ~14"-15" turn 1 and partially ignoring terrain and other models with Shifting earth -> Avalanche, and still have 1 AP and his bonus left that activation.