A worst version of Unleashes boost and stage design, race tracks, mostly nostalgia driven to distract from some odd stage design, extra content isn't much, last boss bad, collision issues, overall pretty short if you know what you're doing.
Colors went worse in all these areas and added wisps.
The fact this games scored dropped from 80 to 72 within a couple days makes me believe that the Mania syndrome only lasted until released with Frontiers. After Mania it had been so long since a good well-received Sonic game that people ignored issues with Mania, the fact it was a reused asset nostalgia pull that didn't compete with the originals at all, though still serviceable. It maintained an 80 score (which no 3D sonic game outside one has gotten) based on that early excitement, but people stated complaining about it more later on and now it's not seeing much discussion or replay, people are going back to the original 3D games, or the early Adventure games or Unleahsed/generations.
With Frontiers, the hype seems to have died at launch, so the score has dropped in such a short time instead of Frontiers locking in at 80 and then people changing their minds on it later. With another 20 reviews or so to go, it's possible this hits the 60's given it's already at 72, that means reviews have been on a down trend after the first 24 hours.