I give the game a 9 out of 10.
As for Sonic's Story, instead of copying & pasting the first installation of Sonic Generations (2011), I thought Sega could've replaced at least three of Sonic's stages with stages from Sonic & The Secret Rings, Sonic & The Black Knight, Sonic Lost World, and/or Sonic Forces along with some bosses from these games.
As for Shadow's Story, it was great! Even though Black Doom was introduced to us in 2005 (Shadow The Hedgehog Game), Sega found a way to make it feel like Black Doom has been around since Sonic Adventure 2 (2001). It was cool to see Shadow being used in a 2D aspect and the new power-ups such as Doom Surf, Doom Wings, & Doom Morph to represent his link to Black Doom.