In both of my latest playthroughs, I've noted a particular hatred between the Vladians and Battanians, to the point where they were almost constantly at war in both campaigns. Balance is pretty good though, except for the Crossbows. They are particularly vicious. I use a Shield Wall to approach but the cavalry hits hard against them on the way in. I out-range them occasionally with my Hero units/Fians but that doesn't always work either. Against Vlandia, I usually win the engagements under 150 men per side, but lose most of the larger engagements involving more of the AI on my side. I believe there is a culture thing going on that makes Vlandia and Battania constantly declare on each other. Aserai and Sturgia can be knocking on their doorstep but they are mostly ignored it seems.