In the game without the lord, they got bogged down in combat with a marine tactical squad on an objective, before being pounced on by terminators. With only 5 bikers, they were unable to kill all the marines before the regular shooty terminators killed them.
In the game with the lord, and I will admit this was entirely my fault, I shot the terminators with my bikers first, as they have a pair of plasma guns hoping to lure them into a combat. I figured Chaos lord with daemon weapon, champion with power fist and 4 other bikers would wipe out a unit of 5 combat terminators led by a chaplain [2x lightning claw, 3x thunder hammers] if they charged me.
It all went wrong as my lord managed to hit himself with his daemon weapon, the unit got ground down and wiped out. If I had managed to actually go first, I am fairly confident my lord could have swung the combat.
Mark of Nurgle is expensive, It may have been better to have Mark of Tzeench tho.