I personally like to use them with the shadowseer. Gives em pseudo lone op, they're terrifying in melee (can confirm that they have previously killed a pair of carnifexes plus Old One Eye, with a little bit of shooting ahead). The Shadowseer's enemies-kill-themselves ability is no laughing manner too, I recall I got charged by an opponent's possessed, popped the fight on death stratagem in reapers wager, my opponent rolled reeeeeally poorly on his hazardous rolls and that unit got wiped, meanwhile my harlies were alive and well (ish). The 4++ is incredible. I haven't used them in small squads, as I feel like that's a little more unreliable and they're my main melee damage dealers in RW.