I'm working on improving my own Coven list at 2000pts, but basically I'm constructing a Brigade:
x30 Wracks in 2 Raiders, and 2 Venoms
x5 Grotesques in a Raider
4 Talos in Squads of 2
3 x5 Man Units of Scourges
2 x5 Man Mandrakes
Obviously, the Mercs help you fill out the Detachment, without taking away obsessions. With some of the new FAQ Rules, like the rule of Three, you almost have to build pure Covens or Cults a very specific way, since you can't just spam Haemonculus or Succubus anymore for various detachments like multiple batallions, unless you're using special characters. Also, Wych cults don't have a Heavy Support Unit, so you really almost have to build a Batallion, Outrider Detachment, and Flyer Wing to maximize Command Points, while still following all the rules.
I think a pure coven or wych cult can absolutely work, but it is by necessity limiting in the amount of builds you'll be able to do. Honestly, with Pure Coven, I'm not really sure if vehicles or an all foot horde style with Prophets of Flesh is more competitive. With Wych Cults, I think Red Grief with 3 Raiders full of wyches, Succubus with their own venoms, As many Reavers as you can fit, and 2 Razorwings, and a Voidraven Bomber will be very good.