We played a test run through with some of the stuff for our Istvaan III game today.
It was mostly World Eaters on both sides but some Sons of Horus made their appearance. We used a scenario from the AoS Balance of Power book.
The Loyalists were heavily out matched. They had 2 Legion squads in rhinos, 2 Veteran squads, a Seeker squad, a Praetor, a Contemptor Mortis and two heavy support squads. Half of their stuff started in reserve.
The Traitors had Angron, a legion dread, a contemptor dread, Red Butchers, Destroyers, a Veteran squad, Rampagers, a Baneblade and a legions squad in a rhino.
The loyalists had to move to the center of the table where the commander could bring on reinforcements if he was able to be within 3" of a communications building.
He was able to do so weathering the full destructive force of the Baneblade for one turn.
The reinforcements themselves were less useful. The Sons of Horus heavy support squad took a hit from the demolisher cannon and fled. The World Eater heavy support squad mixed it up with the contemptor and held on but never fired.
The two veteran squads in reserve did minimal damage before dying to large amounts of fire.
Despite all of that the Loyalists won by not being eliminated by the end of the 6th turn.