[@Lady Selune] Xenos get cool xenotech, and access to literally every Astartes chapter through the DW. The Red Hunters work equally with each of the 3 Ordos, so really Hereticus is the only one without their own Astartes chapter. I'm not a fan of SoB (probably because they aren't my fetish), and the purview of Hereticus is like...

"We hate psykers!"

"Yes and what else?"

"We... uh... We hate psykers!"

Plus they're in bed with the Ecclesiarchy which is a whole other can of worms.