I've been trying to find a point count for the mourngul, does it exist? Couldn't find it in the leaks or the GHB17, but the warscroll is so powerful!
That, plus the CC abilities are just disgustingly good. The support characters that give +1 to hit and +1 to wound will be so good with the battleline, especially the spirit hordes now doing a MW on a 5+. Not necessarily something you want to bet on, but a solid bonus that will help churning through enemy lines. I guess that's why they didn't want to implement it across the board, it would be tricky to balance with the presence of so many buffs.
All in all, if they're good I'm definitely starting an army with them; I think they'll be the perfect way to get introduced to competitive, as they tend to simplify several aspects of the game and focus on table presence.
I started a Tzeentch army in the winter hoping to run a skyfire list, but now the only list that seems viable is a massive horde summoning / magic twisted army that seems super costly and would take ages to paint...