Regarding The Royal Mordants, I play it in Blisterskin and the options offered by a pack of 40 Ghouls able to move 16" are just great. I feel 12 Flayers is pushing it, they are only range 1 in melee and 50mm bases. Moreover, I never felt they needed the double move (But I play them in Blisterskin with Deranged Transformation and/or Spectral Host). I could see it used on a small Flayers squad to assassinate a key hero if a small opening presents itself, but a squad of 9-12 wouldn't be able to fit in a "small" opening ^^.
This battalion can also help the Horrors you are expected to take, they have never been cheaper and pack a serious punch with a Ghoul King around, in Mordants, they gain the mobility to choose their targets.
All in all, this battalion offers options the others cannot. And it is a lot of fun to play !