I feel like I'm one of the few C3 users to make a rhythm game. Although the special effects in this game may or may not be putting C3's limits to the test. Here ya go: youtube.com/watch
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The reason to as why not a lot construct 3 users make rhythm game is because of the music choices. You need good music to make a good rhythm game. Most good music are under copyright and paying a hefty licencing fee for each song could add up. Not many devs are good at making their own music or have the time to make the music and the game at the same time. You can indeed out source your music and have someone make it for you, but that will cost you money. Using public domain music can work too, but it can make your game kind of cheap. When using public domain music in your game, make sure it is the original composer and not of some-else rendition of the music. You don't want to end up putting someone work in your work and ending up having to have to your game taken down or paying the person who made the music you took. This is why making rhythm game so hard and complicated.