After discussion revealing some of what is likely to appear on a table very near me - it seems like Ynnari can pull off some leadership shenanigans rather well.
None of this is really new, DE/Harlequin buddies lists could already do all the basics. What may have shifted is how easy it is in terms of avoiding "tax" units and some little buffs to make it work better.
So the core of this build is the Mask of Secrets on a Harlequin Shadowseer and the Armour of Misery on a Dark Eldar Archon - who probably has the Webway Portal to deliver this to exactly where it needs to be. The Visarch is an optional addon - I will get back to him.
So what this does is give a 6" bubble of -4 Ld which can be exploited several ways. Bear in mind that being placed within 6" they will be perfectly positioned to Soulburst when they kill things.
First and possibly most devastating is the Mirror of Minds power the Shadowseer can get. If that Shadowseer can engineer a situation where the target is Ld5 or less then the target must Deny the Witch or will be killed so long at it can be killed by wounds. With the modifiers above anything Ld9 or below may as well not bother having wounds/saves/FNP because faced with a "keep rolling until the model is removed" effect the model will be removed. The Ynnari can double the chances of getting this power if the Shadowseer is the warlord because they get re-rolls on the warlord trait and one of those grants the ability to simply choose psychic powers rather than rolling for them. Riptides and Stormsurges are going to be the primary victims of this power I think.
Psychic Shriek is the other psychic use that i think we would see a lot. Tau do not generally have the best Ld so taking a -4 before being hit with Shriek is nasty.
Another nasty one is the Death Jester. If s/he inflicts a wound the unit will be taking a morale check at a cumulative -6 or fall back in the direction of choice of the Ynnari player. Then if they charge that unit it has to try to regroup at -4 to Ld or it will simply be removed from play.
It is possible to go all the way here with a Coven Coterie for the "tax" cost of a couple of wrack/grotesque units. The Ynnari unique HQ characters can go here without losing their Strength from Death rule. Those HQ (and the tax units) have a further -1Ld effect in a 12" radius. At this point even Ld10 targets will just die to that Mirror of Minds power and one more wound per Psychic Shriek will really add up. The Visarch also has a rule that means any unit within 3" uses its lowest leadership rather than its highest, when Shrieking a drone controller commander with a swarm of marker drones this would mean taking 3 more wounds per Psychic Shriek.
As countermeasures I can only think that the Talisman has some value - every Shriek denied might save a lot of models. Beyond that I think I will need to maintain my usual extreme levels of Interceptor fire, the Ynnari will probably want to use perfect deep strike to position the rather small 6" super-lethal range of this.
So bring on the scary clowns.