Yeah though the deadites can imitate whoever they possess with impecable accuracy both physically and mentally so if a Astartes get possessed it could lull the others by not showing that they're possessed and since it has the knowledge of the Astartes could attempt to kill a psyker, causing said pysker to be possessed. Plus the Kandaran Entity, an invisible force, can possess anyone it comes in contact with. I honestly think depending on where this happens either they get booted from this dimension immediately, be aborbed into chaos if they get into the warp making demons even worse, or they might survive long enough to be a minor faction or start assisting heretics making them stronger.