Sonic 06 had the concept right, if not execution. Its problems have been documented extensively by many. I'll always remember Slowbeef and Diabeetus suffering through that game. As much as Slowbeef sucks balls at video games (he will admit as much,) there were many unforgivable acts the game committed against his sanity.
What 06 did right:
-large scope
-big ideas
-fully 3d
-colorful
-fast
What 06 did wrong:
-collision detection is a joke
-manual saves only -> game over resets to last manual save -> it's possible to lose an entire save file if you game over early
-physics don't work
-spaghetti code
-the concept behind Silver's gameplay is fucking stupid
-Tails' ring bomb having the same models and sound effects as actual rings dropping out of your character is infuriating
-Knuckles and Rouge gameplay are jank as hell with the lack of working physics
-Shadow's vehicles are broken
-snowboarding is broken
-bosses are broken
-Amy's gameplay is shit
-too many playable characters in general (most are bad)
-2/3 of the game is repeated content and forced filler with locked rooms and enemies you are required to kill
-all the fire areas look ugly as sin
-playing 06 "well" seems to be largely a matter of moving from scripted point to point and letting the game play itself... When those very elements aren't breaking and sending you through the floor
Would 06 be a good game if it weren't rushed? I'm still going to say no. Mainly because Silver's gameplay is fucking terrible and both he and Shadow retread Sonic campaign. Best case scenario is Sonic Team would have deleted Shadow and Silver's campaigns to make Sonic's longer. But I don't think they ever intended to do that, even in a world where '06 had infinite time and money. They probably would have fixed most of the collision issues, but not the lack of momentum for characters. I don't think the sheer amount of playable characters in 06 would have worked out regardless. There are too many and most of their special abilities are stupid or jank. To the point I don't know what they were trying to do, in the event they could patch the game to realize its full potential. Tails' ring bombs are just annoying and stupid.