Majority toughness 3 for Kharn isn't good. Cultists die
fast, even to bolter fire - If you meet an
IG player with 3 Wyverns they're dead. To full utilise Kharn, it's better to include Cypher (and maybe a Sorcerer or two with Invisibility) and give him a squad of something dangerous in a Spartan Assault Vehicle, aka the Kharn Party Van. Sure, it's expensive, but Kharn, Cypher, and delivery method is wasted on Cultists.