That could be.
Sometimes critics and fans have different mindset.
Its gameplay might be classic 2D sidescroller but it was fun to play, as fun as Advance series, Rush, Rush Adventure, and Colour DS.

I just didn't like that the modern stages were just as much side-scrollers as the classic stages.
Now, given the N3DS version of Sonic Generations was for a handheld console, if they used stages from Game Gear, Advance and NDS titles then I would excuse Modern Sonic stages being side-scrollers. But they didn't, they used main console Sonic games.
And rendering limits is no excuse given games like
Kid Icarus: Uprising shows the rendering pwer of the N3DS at the time, so it wouldn't be that impossible to make full-3D Sonic stages on it, even if not as high-rendered or HD as the Wii U or other consoles at the time.
Hell, you can probably fit most Dreamcast games on the NDS.