It's very nice seeing a game that takes player-led design and tries to add difficulty to it. While I'm sure there were other ways of doing it, the use of music in Song Runner feels like such a cathartic option. By creating a situation where the player has to correctly time their button inputs so platforms can be in just the right spot, it means that there's never a moment where the player feels like the don't have an idea for a way forward. You always have the full picture, and when something doesn't work, it's just as easy to switch perspectives.
Despite giving the player such control over the level's layout, there is also still a pretty well handled difficulty curve. Increasing tempo in a rhythm game is always the first thing you can do to make it more challenging, but the good thing is that the tempo never increases too drastically between worlds and doesn't get too fast by the final world. On top of this, the game's hazards are all pretty well crafted, with each of them creating a new challenge for the player to overcome, but always having a clear way to interact with the platforms that they player creates.
One side note as well, the sound design is pretty phenomenal. Between the different themes for each world, the muted music when in a "No Music Zone", and the ability to listen back to the song you've created throughout the level with some extra tracks added on top, the amount of detail is very highly appreciated and is a great example of how to add depth to a game about music!