What I don't get is why does Sonic Team build great engines and then ditch them for horrible ones.
Why not use the Sonic Advance engine for Sonic 4? It's nearly identical to the Genesis games, but instead, it looks like Sonic 4 is using the Unleashed physics, or something entirely new.
Same with Sonic Adventure 2. They had great controls for that game. But in Heroes, they ditched it for the Slip-n-Slide-O-Rama™ engine. And in Sonic 06 with the Backwards Physics™ engine.
It makes no sense. When you have something good, just use it!