The demons are great.
Their slaughterhosts are great, their battalions provide for a mix of units.
Most of their units are good and even crushers while being weak are cheap enough to spam for mw output.
They have nobrain power strategies of thirster battalion, "fight again for CP" ability and cannons+mongers.
Mortals are weak.
They are slow. Demons are slow too, but mortals are slower.
They have weak scaling with offensive buffs because of low model count. Not attacking in two rows is a weakness of Khorne in general, but at least letters have lots of bodies.
They have very weak battleline. To the point it may be better to take doggos/letters even in a mortal slaughterhost. I think 1 warriors unit (for a battalion) and 2 doggos is my default mortal battleline for the near future.
Things that I'd try to make mortals work:
- skullreapers in anti-rend battalion. Skullreapers are interesting unit because they don't need any support. Not even a bloodsecrator really since he has very bad scaling with them. Reroll hit, reroll 1s to wound in a host, no morale, can kill tough things with mortals, can kill horde things with attack volume. Giving them anti-rend and priest will also make them quite tough. Tough enough to carry the whole army? Who knows.
- wrathmonger spam. They are the best thing per point that mortals have, 1 10man unit basically has output of 30 letters and costs slightly less while having some upsides over letters. They are also the only dudes who can attack in multiple rows or attack through your other dudes. They also have a good battalion and a good exclusive CP ability from Skarr.
- Korghos Khul. While playing Goretide you're basically rerolling 1's on hit/wound with him and host bonus. That should be great on our 3+/3+ dudes which are our main damage dealers.
- spamming priests. Priests are pretty much our best support heroes and unlike demons mortals can have multiple priests in a battalion.
- skullcrushers. You can apply skullreapers reasoning here except they are faster and don't need anti-rend because they are already 3+. What I think is their main upside is mortal output outside of combat phase since entering combat phase gets more and more dangerous nowadays with more fight first stuff around. Also they can charge and vomit mws with a 3 blood point ability, that's cheaper that fighting for 4 and has longer range.
Then there is always a possibility of playing demon slaughterhost with bunch of letters and skullreapers/skullcrushers as your second wave. Or thirster battalion with a fight first artifact + some mortal battalion. The upside of playing mortals is that they won't get shredded in shooting phase.
StD future release may improve mortals wherever it may happen. I wouldn't count on that though since GW puts all of the good stuff in allegiance bonuses nowadays.
On a side note slaughterhost bonuses are pretty much mandatory. None of the base command abilities outweight what slaughterhosts provide. If you're not in slaughterhost you're actuvely harming yourself.