I play Incubi, Mandrakes, and Khymerae/Clawed Fiends for combat. But the only unit I really expect and get solid results from? The Incubi.
Beasts have a large footprint which can help a transport or group of transports that are moving up a board edge and just need a flank covered, screening deep strikes and charges is really their big thing for me, the fact that they eat low armour infantry is just a bonus.
Mandrakes can do some light fighting and better shooting but are also good on turn 3 to drop in and try tying up vehicles and even targeting any vehicle on its last leg with the Baleblasts. But I don't find them really reliable in a combat role unless they were picking off some stragglers. I see them more as a disruption when they're in low numbers.
Incubi, on the other hand, are blenders, and the Klaivex's occasional ability to do buckets of damage just helps them mulch characters. Pretty fragile even with the 3+ so just know what you're trying to achieve and whether it's worth it when you let them out of their transport, they don't cost a lot but if they're used carefully they can get a lot done.
I tried my old Coven stuff earlier in the summer and I found it just didn't really have the same oomph as it did before, but at the same time when I'm playing a list that isn't Warrior spam a lone unit of Wracks will probably get more done than a lone unit of Warriors, so I might try them out.