Coven can be fearless very fast ,,, that is their main benefit. Haemie in a coven list really does need any artifacts.
Grots and Wracks - fearless - are for cc and holding objectives - generally need transports unless you want to DS a squad of grots with a hamie - squads of four with aberation are pretty darn good.
Dark Artisan truly rocks. I've used it. Dropping a haemie w/liquifier, a talos w/ sc and ichor and a cronos in someone's back deployment will ruin their day.
Now taking five Talos is something I also tried - it worked out very well for me. I opted to scout the buggers and they pretty much controlled the table with 30 splinter shots a turn. It is very expensive though. You are creating a no enter zone of over 16" circumference - spread them out as much as you can. No one wants to be assaulted by that squad. That is a large area that you can be in turn two at the center of the table.
Now a little bit down their is a nice breakdown of the various coven detachments ... https://www.thedarkcity.net/t10420-play-testing-coven-a-review
No need to go into details here and rehash it.
Paper is overpowered. Scissors is fine.