I guess it worth looking at the f10 forums, ASS was introduced to the 5 series shortly after it was introduced. ( had it on my 2012 model)
I have personally not read if any issues people have had there as regards starter motors. These guys would have the age and mileage under their belts now so would be best placed to highlight that. As I say I’ve not seen anything on that personally not to say that there isn’t .
Not sure about the engine itself. I’m sure mechanics here can help out
I’m sure it save fuel if it’s used where you stop for x seconds or more. The value of x I don’t know
This video may be worth a watch
https://youtu.be/dFImHhNwbJo?si=3oJHRwLKikq5jVZh
Personally I disable it as I dislike the lack of refinement about it on my m550. The hybrids I suspect would be smoother