Yeah I've found my Grotesques even against Guard not great. Yes they kill Guard in droves. But for 100pts less Incubi could do just as well.
Regardless, my dedicated combat units are ultimately left stranded by units that Fall Back and then promptly turned into pink mist.
I'm thinking I'm going to abandon the Coven and field mostly Kabalites in vehicles with maybe one dedicated combat unit like Incubi with an Archon to mop up stragglers and make it tough to push off an objective. A tooled up Archon with 4 Incubi run less than 4 Grotesques, which has typically been the optimal number in their Rampaging days