You do eventually see them, but the Rulers tend to show up rather late and rather infrequently. I get the feeling that the LW folks would have preferred if the DLC could just go away.

It's strange. I had my Rulers mission show up .... although after completing it I saw it ♥♥♥♥♥♥♥ reappear. Then it reappeared again. When I tried it the second time the Viper King was there but ...... completely untargetable and unresponsive to the game. I decided to just not do the mission the second and third time but yeah.

Had the serpent king mission 3x so far, bugged bad now that he is dead though... finished the first 2 objectives, but there is no serpent to fight at the end. Am unable to call for an evac.
I think I had 5-6 regions before he first popped up. Was pleasantly surprised at the reigning in of the "Ruler Actions" it was ridiculous to reload your gun and give him an action.

I don't think pavonis even wanted this in the game. I have heard if you disable it (perhaps some mod launcher?) then it works and if you 'enable it' it does not. Its certainly not intergrated into LW2 very well.