So much hate, I can sense it flow through you...
Ynnari (Harlequins) are my second 40k army, because the hipster potential of Necromancer Space Elf Clowns was too great to be ignored. The main benefit I get from the new rules is the opportunity to be faster and break counter attacks. Neither of these adjust the fact that I am expensive and only have a 5+ save. All three GS2 characters are overpriced, and the formations are kind of terrible except for the reborn warhost.
All that said, what I am trying to do is maneuver to break your center as fast as possible. I plan on you using conventional deployment methods so that I can dissect the force quickly and minimize response. That changes things around a bit, as the more elite your army becomes, the easier it is to do. If you run a true death star, then I only have to maneuver around about half the units. If you run grav, my 5++ and lack of armor makes it into a severely overpriced (worse than) heavy bolter.
The things that would give me a REALLY hard time:
1. Anything that ignores cover. If you take away jink and cover, my expensive troops go down to a stiff breeze.
2. Spreading out while maintaining overlapping fires. In order to use soulburst, I need to run units in groups of 2-3. The more I can clump up the more I can make supporting soulburst actions. If I am forced to spread out then soulburst goes away, and I can't hide from your supporting (long range) fire.
3. Bolters in drop pods. Yes really. Ten tactical marines firing rapid fire can easily destroy any unit I have. They are also cheap, so make it hard for me to get that 3 unit to 1 target ratio I'm looking for.
4. Counter my psychic phase. The only power that I rely on is the soulburst action. If you stop that, you've screwed up my plans.
Things that will give you a REALLY hard time:
1. Webway Assault / portals - I can drop units on the table without scattering. I would counter by not leaving easy targets on the table and putting units in reserve. I also have no way of delaying reserve rolls. Its a 5-6 turn game, and attrition is in your favor.
2. Chaining combat. Soulburst allows me to charge in to one unit, kill it, then charge into a second unit in a single assault phase. I would counter this by leaving 12" between units so a second charge is not possible.
3. I have a lot of S6 shooting, so I don't have a hard time with rhino chassis. This is another reason to consider running drop pods instead.