Star Forts Useless?

[Ardri]

On offense is just amazingly hard to make your units move inside the Fort once you open a breach.

I haven't really thought this is too much of a problem in my experience. One of the best ways to move them inside is just to have them garrison the available buildings inside the walls of the fort. The AI in my experience never occupies them and once your men are inside they have basically a 360 degree firing radius, not to mention they decimate the AI units standing in the center square.

On defence it's simply too hard to predict where the enemy will come from and how to hold him back.

I agree fully on defense. You spend a lot of time with units sprinting around the top just to watch a unit of the AI go and stand at one of the corners and not do anything. Defense isn't difficult, but it is annoying at times.

[Sheogorath]

The attacking AI is really weird. Does it tend to do a full circuit of the fort before assaulting for anybody else?

I had one unit of AI cavalry that apparently decided to reenact the Illiad minus dragging Hector behind a chariot.