i dont know if id say they leave a "rough" surface. The do leave a surface though, which is actually how they work so well at removing aluminum oxide. Al oxide is a hard brittle material, but only a few microns thick, the impact of the beads breaks it up and it chips it off, but really doesn't remove a measurable amount of material.
I'm certain walnutshell would leave a smoother finish as its softer media, but I don't know if it would be able to break up the oxide well. Also, adjusting air pressure gives you a wide range of results for any specific media.
Lastly, I can't think of an aluminum engine part I would be worried about hitting with glass beads. Certainly anything cast will not get any rougher, and anything machined ... I don't think the tolerances would be altered any just the surface texture.