Apparently, Caipha pairs well with Word Bearers. Aside from the occasional loss due to missing cards, I lost badly to Hibou Khan. I don't think I've ever won a battle against this warlord, no matter who I play.
Played with Sons Of Horus, as you can see. There is no special synergy. Sons of Horus provide us with Astartes overstat and sometimes Hope's End and Chieftain Squad, a lot of Front Line to protect the health of our warlord. Some fights were quite long. And yes, Erud Van is perfectly combined with the White Scars vehicle and the mission, it was really painful, although I didn`t live to see the mission.