3 words.
Worst level Design
Barely any detail there, it's all about using the terrain tools,
That isn't free roam, that's pure randomness with assets added to make it look like a "stage".
Also if someone makes a fan game, fans usually expect quality, Not a random terrain with textures painted all over with the brush tool , and some assets added to make it look like something it isn't.
But nowadays sonic fan's are pretty much amazed with anything they can't do.
It's always further more rewarding creating something from scratch, then using something already made.
Anyways goodluck on programming the engine.
Hoping to see more .