Finally, the video I was waiting for!
Obviously Ultimate looks better in pretty much every conceivable way over the Wii U and 3DS versions, by a large margin in-fact, but there's a few interesting nitpicks here:
-Smash Ultimate apparently has the worst input lag of any of the Smash Brothers games
-Smash Ultimate's Donkey Kong model may be slightly less demanding given the fact that it is no longer trying to replicate real fur, most likely an artistic choice
Also I seem to remember seeing a video explaining that Gaur Plains was missing some shadows from the Wii U version.
Either way, it looks like the improvements in the most important areas were made!