I was lucky enough, as I was still working for Yamaha, to see a very well made and interesting video of the process used to make Yamaha's S950 Audio Drum styles.

You are totally correct...it is far from easy.

Yamaha even went to the trouble of using the same drums, mixing board and effects units, as well as recreating or using the original studio environment.

And that was just for drums.

Ketron goes a good deal further, and does much the same with bass and guitar parts.

The styles sound like a "real" band, because, essentially, they are a real band.

Ian