Not only is it visually stunning, but it seems to pick up on what made the Mega Drive/Genesis games so popular: the physics engine. Everything in the game is momentum-based, and I think that's what we really need in a 3D Sonic game. In my humble opinion, this is how Sonic should have transitioned to 3D.
The "Adventure" games had that style of 3D gameplay. Had most games back then been built with more powerful engines, it probably would've held up a lot more.
That's somewhat true, but it also started introducing elements that would be abused in later games (things like splines/automated sections). I always felt that these elements robbed the player of control, but I will grant that some things are not easy to implement in 3D games, notably loop-the-loops, without some form of automation, and this is also a bit apparent in Sonic Utopia, as players frequently lose momentum at the top of a loop and fall down. That being said, the developer(s) did say that this will be addressed in later releases.
Linearity is also apparent in the Adventure series, especially the second entry (and I guess mostly any 3D Sonic game); once you've gone through a section of an area, you can't go back to explore for goodies, you just must keep going forward.
It's also apparent that Sonic Team tried to do too many things in it... Sonic's levels, Big's fishing stuff, the treasure hunt stuff... Even in S3&K and Sonic Mania, despite having multiple playable characters, the underlying theme is the same: a fast-paced, momentum-based side-scrolling platformer, and each character's abilities can be used to explore different sections of an act (or the actual act is somewhat modified to suit your chosen character).