Physics and control differences between Unleashed and Generations will always make it feel different to play
Certain stage gimmicks (like QTEs) are not replicated since it's mostly limited to the things Generations already has
There's no 'game', being a mod the only thing they could do is add the stages to a custom 2D hub like in Generations. Of course, it wasn't without some issues: The go-to way to 'play' it for years was the Unleashed Project, an incredible mod that recreated the main acts of the game within Sonic Generations. As with other Sonic games from that generation, Unleashed never recieved a PC port.