I don't know if it was intended, but I'm noticing a large bias in favor of the defender in autocalc battles... It came to my attention by watching, via spies, two rebel settlements (one in Iberia, one close to sweboz lands), which the AI was trying to take. In both cases, the AI attacked the settlement with a full stack army, then another, then another, and they were unable to take the settlement. the defenders suffered losses, but the attacking armies were destroyed.
So it happened that I led an army myself against one of these settlements (the one in Iberia). I had an army with some 1600 men (mainly lybian spearmen and iberian scutarii). The defenders, at that point had a handful of units, down to only a few men each. Some 200 men total, I think, mostly basic troop types (many of them had gold chevrons by then, tho). The city walls were already damaged by then, too. so I decided to autocalc just to see how things would go. And my army lost, clear defeat. I reloaded the game and hired two mercenary scutarii units; now it would be 2000 vs 200. Clear defeat again.
So I reloaded the game again and decided to have a go at it, without hiring the mercs. I made a few rams (strangely, I was unable to attack a part of the wall which was already partially broken with my ram - is that a vanilla bug?), I tore down their flimsy wooden walls, infiltrated with half my army, and took care of the few men there in no time, with some 70 casualties, at most.
Later on I would place a spy in another eleutheroi settlement and watch the sweboz try to take it in vain, several times over. The bottom line is, I suspect the defenders might be getting too high a bonus in autocalc, even with weak wooden palisades and just a few basic troops, and this might be making it too hard for the AI to expand into eleutheroi lands. It doesn't really affect the player, who will usually command the battles personally and crush the weak defenders. This makes the game even easier for the player, who can expand quickly into rebel provinces, while the AI must expend several armies to do it.