1. Maps like Green Hill and Chemical Plant are used yet again for no reason involving the story and are visited too many times
2. Other characters also appear but are never actually used for anything. For example, Chaos from Adventure 1 appears but is defeated by Classic Sonic in a cutscene; you never fight against him.
3. Much like Sonic Colors and Sonic Generations, the game can be completed in about 3 hours if you don't go after collectibles.
4. If you die during a SOS mission, you automatically fail the mission and can't try it again.