So I've been running the Daemonkin ritualist specialist detachment quite a lot recently and here is what I have discovered so far.
Sorcerer with jump pack, delightful agonies, warptime
Dark Apostle+disciples, -1 to hit prayer(something something illusions?)
2 Greater Possessed. Both mark of khorne. Sheppard of the True Faith on one of them.
15-20 possessed marines, mark of slaanesh.
1. These guys can take a beating but only if you list plan for it. I have run mine as Red Corsairs with Mark of Slaanesh and a dark apostle, sorcerer, and MoP for support. Against imperial guard they have survived up to turn 3 not holding back at all on multiple occasions. Other words, do not bring 15-20 possessed marines without support characters. That always ends badly and ususally on turn 1.
2. The MoP sucks. He is too slow and his buff powers too short range to be useful. The possessed blob is fast and has to be a threat to charge on turn 2 at the latest and preferably turn 1. As such, you just cannot keep up with the MoP so he is a total waste of 98 points imo. Furthermore, cursed earth is too unreliable at warp charge 7 to be of much use. A sorcerer with delightful agonies and warptime is vastly superior. Getting stuck in on turn 1 and 5++ FNP is superior to anything the MoP does. I've run him in 6 games now and he has never been a key player. He either failed to cast his spells or just fallen behind the blob. IMO he is the least useful buff character for possessed blobs. He might still be good for summoning or supporting ranged daemonkin but I have yet to test that.
3. Greater Possessed are great. I would take 3 in every list if i had the models. These guys are so good for 70 points i dont understand why everyone doesnt have them in any random list. The thing I like best is that you can give them the Daemonkin Ritualist trait, Sheppard of the True Faith to give your blob some much needed punch in addition to making your character that much more dangerous in close combat. Always make these guys MoK so you can fight twice if needed. Always bring at least 1 and give it the detachment warlord trait. Tactically speaking you can do a lot with these guys. I have used them for their aoe str buff when needed. I have used them to character snipe when getting lucky 6s on advance rolls. Nothing is better than advancing 13 inches and charging a farseer or company commander and ripping their face off for Slay the Warlord. You can also use them to capture objectives on the bottom of turns with a well timed charge+Fury of Khorne.
4. The Contorted Epitome is a very good ally. If you run daemonkin ritualists you should look into daemon allies and nothing is better than the new Slaanesh Mirror/rorschach test model. It can easily prevent fallbacks, casts hysterical frenzy with some reliability, hits hard in close combat, gives you another +1 str, and it is an excellent psychic defender. If you face a gunline this model will cause your opponent to lose their mind after the first time you prevent them from doing their favourite "tactic" of falling back and blasting you off the table then bragging about it later on the internet. Be prepared for a table flip if you cause one of their flyers to crash because they cannon fallback during their movement phase. My prefered flavour: Outrider Detachment: Contorted Epitome, hysterical frenzy, phantasmagoria. 3x1 units of Fiends of Slaanesh.
Overall I think the Daemonkin Ritualist detachment is fairly solid but only because of the warlord trait it gives. Possessed marines in general are pretty good in a semi competitive environment but only with proper support.