Personally, I find that Coven units have great potential but are quite specialised, so I only use them as one part of a balanced force.
Wracks, for example, make good troops for taking on high toughness enemies, and a liquifier gun is good for melting through power armour 50% of the time. However they have no ranged capability beyond "template" distance and don't deal well with hordes or armoured foes in assault. I like to have a few of them among my Wyches and Warriors.
Grotesques are good for being big and scary, and drawing fire away from your more fragile units and transports. They are pretty formidable in assault, they aren't too worried about overwatch due to their toughness, and they have the strength to threaten any vehicle with a rear-armour of 10.
Talos and Cronos have great models and are fun to play with. However, they can be a bit vulnerable as they are not tough enough to shake off anti-tank weapons and they have no invulnerable save (until you get a pain token). They are quite slow, especially as all of your other units will likely be in transports. Also, they take up HS slots that you could do with using for Ravagers, given that a coven list is already massively short on ranged weapons.