It's understandable that seeing games that are "selling well" is a way to determine if C3 is a professional game dev software, but that could give many different results - What if someone produced a really generic basic looking game, but it's generating a lot of money due to the developer designing it to be addictive? What if someone made a beautiful huge RPG game with many complicated systems, but they failed to market themselves well and aren't making as much money compared to some random game made on Unity?
I'd frame it more like "How accurately can C3 produce the idea that I am thinking of?"
What project idea have you got? Is it a huge RPG game, or a basic idle game that is very simplistic but addictive?
Do you want to produce games for Windows/Mac/Linux? Android/iOS? Console?
Whatever your idea is that you want to create, ask about that.
I am someone developing for Windows/Mac/Linux, on a variety of different projects (platformers, rpgs, rhythm games), and I can say that I'm having a GREAT time with using C3 and the results it produces, even when it comes to performance.
As long as you have an understanding of game dev logic (the same game dev logic that applies to ALL game engines, such as Unity). Game dev logic such as "don't put a shader effect on a sprite that you will have 100 instances of (instead, put the shader effect on a layer)" or "Don't test for collisions endlessly for many instances" and such, then you will have a smooth experience, I can assure you. To be honest, if you're targeting Windows/Mac/Linux, you can get away with these "game dev logic" issues a bit more, since PC's are so powerful.
I'm less familiar with the experience of Android/iOS, and I am aware console support is a bit eh as of now, but maybe some others can chime in with this.