Just by my opinion, i think the dislike for sonic heroes, is probably the weakness and flaw that it has. Due to the way it was programed in nature, that allowed for it to be ported to multiple platforms instead of just one exclusive like Game cube or something, it suffered a lot and on various situations. The ps2 is probably not the best which i played, and just from experience alone, is very repeative considering to complete the game you have to use all 4 teams and while most play the same, is basically a difficulty selection, sonic being normal, Dark being hard, Rose, being easy, and Chaotix being some sort of puzzle solving. While it incorped unique personality and game play for each it wasn't bad in gameplay but it seem much unlick a sonic game with some pausing to solve some situations, not to mention the glitches.
The game personally has some good and bad points, the one thing i don't like is how it opened up this Skill system used in other games regarding the Speed flight and power system. While sonic advance 3 wasn't as bad using it, just made it seem a bit bad not knowing which combination of characters to use for effective game play, and sonic riders having it made some characters seem better than others since each skill had a shortcut that was exclusive to them, that the character stats and gears hardly mattered with just using shortcut is hardly much skill to the gameplay. Every since they stop making sonic games featuring multiple players it really hasn't be a thing.
The thing about shadow the hedgehog that turned most people off was the voice acting was replaced with the english dub sonic X cast rather than use the sonic heroes cast. Once again the game doesn't feel much like a sonic game but it was still fun, like sonic heroes i say it has ups and down. There is much worst sonic games and hardly these two end up on those list. The multiple stages are nice and the influcence of dark and hero just made it based off sonic adventure two chao garden.
Is fustrating in that some objectives aren't fun like "Destory all enemies" Or find something or what ever, Griding and wall riding is fun, even though grinding has lost is challenge factor that Sonic adventure 2 made grinding famous for in a sonic game, using some weapons are nice and the different transportation to ride, makes me think they were trying to make sonic a halo game. The fun in the game focus more on gameplay rather than story and not all stages are constructed badly (I seen worst remember knuckles chaotix?)