Pretty sure the R8000 main benefit is it can take a bigger cassette because of its extra B link thing

It also looks cooler

Shimano claim the shadow thing (as well as protruding a bit less) has “single tension” whatever that is which will stop the mech hitting the chain stay over rough ground.

My R8000 got stuck on the inside of the chain stay and ended up gouging a hole in the carbon when I hit a nasty pothole. So I would dispute that.