I've noticed that AI doesn't use the new stances.
In particular it would make sense if AI and players have to choose the right stance depending on the terrain.
An example is the prone position, if you're prone in a descent and aim high/far the character is going to assume an unrealistic/funny stance, same while sitting in an ascent terrain.
That could be fixed by limiting the vertical axis, so that you have to adapt your stance depending on the terrain and your target position.
I hope my point is clear, sorry about my english.