[jhasse]

The game includes garphics which wouldn't be possible on the Mega Drive / Genesis. For example alpha blending or lots of sprite rotating.

Also Sonic on the Mega Drive had multiple framerate drops, e. g. when you lose lots of rings.

[TulliusCicero]

It would've been possible on the Saturn, and on the Dreamcast at 60 fps, easily. How much more powerful are even mediocre modern PCs compared to the Dreamcast?

[Lorkki]

Saturn would most likely choke on memory requirements, given how much is going on in the levels, as well as lack of transparency support. Symphony of the Night was the state of the art 2D platformer for that hardware generation and it has frequent loading pauses, despite being much more static.

Dreamcast would probably work, given enough effort by people who know the platform inside and out. But then, a much lower barrier to entry is probably a significant reason as to why Sega was willing to fund another 2D Sonic game in the first place.