I do agree that the nature of the battlefields rendered in the game now is far more complex than in earlier versions and that will cause the AI problems. Buildings (both garrisonable and simple obsticles), fences, walls, wooded areas, high ground, impassible cliffs, ground type (mud, grass, etc).
In STW there was High Ground and Wooded Areas...
However, if you chose a simple map and equitable custom battle on pretty flat terrain then complexity of the battlefield should not a large factor. Following this we do still see the Battle AI doing odd things.
Of course the units are more complex, both in their capabilities and in their extra abilities, which would mean different activities based on whether a particular feature is available to a given unit at that time.
I think it is fair to say that the complexity of the Battle AI has not kept up with the complexity of the battlefield and unit capabilities in the game series.
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