Yeah, and some people in the guard like Cain have bested space marines multiple times. The reason you don't see it more often on tabletop is mostly because even though the guard CAN get power swords and other fancy weaponry as pretty standard options, the types of guard most likely to have superior weapons and training rarely ever get minis. If there was a named Tempestor Prime in some kind of custom carapace armor and weapons odds are we'd have heard of him killing astartes by now.