Well, should we discuss combat doctrine?
I was thinking that the War Bearers would be prepared for anything that the enemy might throw at them, while at the same time focusing an having the right "tools" for the job.
Right tools for the job: The War Bearers follow the Codex Astartes very closely, as it allows maximum flexibility.
Be prepared: In a standard organisation chart, a War Bearers army should fill at least one of each slots under Elite, Fast Attack and Heavy Support. These slots can be filled by allies if the rules allow it. For example if fighting against Chaos, a War Bearer army could include an Ordo Malleus Inquisitor.
Listen to the expert: Due to the War Bearers pragmatic view of things, putting a lot of weight in strategy and tactics, 1 HQ slot should always be filled by a Space Marine Commander, who is flexible and a master tactician.