The level design wasn’t the problem with Forces. Although it is mediocre, it still feels like a Sonic game and that means they got the job done. They just didn’t improve upon the formula, that’s it.
I disagree with this actually. Forces was an active step backwards in regards to the existing gameplay styles that returned; Classic and Boost. Aside from Classic's physics being way more jank than they already were in Generations, the level design is generally much more simplistic and doesn't really cater to the Classic philosophy of earning speed and maintaining it. Generations at least tried to fake it from time to time, Forces generally feels like the designers didn't know what they were doing or were being rushed.
Boost gameplay isn't really too terrible at first, but as you progress through the game the level design just gets worse, and it also gets really noticeable that the drift isn't in anymore. People often complained about Unleashed being "Boost to Win" back in the day, but aside from some sections where going too fast will run you off a cliff in the 3D sections that does actually tend to be a pretty apt phrase to describe Forces Boost gameplay. The physics are also janky, apparently the physics issues are because the physics engine is built off Lost World instead of Generations, but I don't know how true that is.
Physics, character control, and level design all work hand in hand in regards to the overall quality of a game's design, and all three of these aspects are worse than they were in Unleashed, Colors, and Generations. Even if the physics are actually built off Lost World, Lost World was going for an entirely different type of gameplay, and generally played just fine aside from the occasional parkour related problems.
Overall, I don't really think Forces deserves any slack for doing what is effectively the bare minimum to achieve what it set out for. It's a mediocre game, but painfully so, ranging from tedious to just boring. I actually get the feeling that most of the dev time was probably spent developing the Hedgehog Engine 2, which would explain why the overall product feels so rushed.