I say this as a lifetime Sonic fan: literally what were they thinking?? This game suffers A LOT from a lack of direction. In most previous sonic games, stages follow a story and Sonic has a clear, narrative-driven purpose for where he goes and what he does. Here, stages have nothing to do with the story. Things just happen. A minigame randomly gives you a chaos emerald or one of the many arbitrary resources/currencies you need to progress. It really feels like the Sonic Team just made a bunch of cool stages and hired an inexperienced game design intern to figure out how to put them all in a game, and they somehow landed at the most uncreative progression system I can imagine.
The open world does not feel freeing. Sure, you can run anywhere you want, but the actual things you do in the world (boss fights and platforming) are so telegraphed and unidirectional that it often doesn't feel like I have any control over Sonic at all.
This game has possibly the most depressing beginning of any Sonic game, and not even a cool & edgy kind of depression. It's just bland. Instead of breaking out of a plane or destroying a fleet of spaceships with fast rock music in the background, Sonic just gets teleported to a sad, rainy island, aptly accompanied by stagnant and boring music (not to mention the weird changes to Sonic's voice). It actually feels like a funeral for the franchise. Sonic is supposed to be a personification of the wind--fast and energetic and free--and every Sonic game I have played before this one has shown this in its intro. I am currently on the third island and I haven't seen anything that justifies this abysmal artistic choice. Once on the island, you defeat some easy enemies (very anticlimactically) and start collecting some items while being interrupted every 10-20 seconds by actually useless prompts that repeat something a character tells you immediately after they say it.
The default music is so bad and uninspired that they had to give players the option to change it with songs from previous games (after you find them). I never thought I would see a major Sonic game with bad music but the music in stages is always soulless electronic music. Songs in titan boss fights are good so far, but these are few and far between.
Movement is very janky, especially in stages. If you try to move backwards in a 3d stage, or in literally any direction that isn't exactly the path that the devs had in mind, you will die. The camera movement sucks and you often cannot control it. Combat is simple, easy, and boring. Parrying isn't time-based so you can just hold it and wait for the enemy to attack (as you have to do in the Wyvern boss fight because its a visual mess and you can't tell what's going on). Otherwise, you can spam the same button or do some simple combos without worrying about reaction. I have the game on hard mode and haven't encountered anything difficult, just frustrating due to poor design choices.
As of the 3rd island, each island looks uglier and more soulless than the previous one. Sure, its realistic, but it trades any hint of personality for vast expanses of detailed gray and brown rocks. Unless this game has an incredible and perfectly crafted ending that ties all of these seemingly lazy decisions together, I have no idea why it still has a positive rating. I love this franchise and I hope they either get a new director or give them much more creative freedom--whatever would prevent another aimless mess like this game in the future.

This is just horrible game design all around.
The game starts with a 3D version of the typical sonic map, was fun and if the whole game was that way itd be amazing but its not.

Pause, notice, pause, another notice, pause, another notice x10000 my god is this jarring its nonstop yanking you out of the game "LISTEN! READ THIS NOW!" menus are janky as hell, I had to fight it just to change volume.
Could the gameplay be good? possibly, story could also be good but i want to play a game, not be ripped out of the game every 5 damn seconds for un-needed info, its so jarring it destroys the flow, there are better ways.

good bro