Sorry to hear that Juice FX didn't live up to your expectations. You can absolutely request a refund to Itchio support and they will proceed without any trouble.
As for the app, I'm not sure about what sample projects you are talking about, as Juice FX doesn't include any. It does include a handful of presets which I've just gone and tested and they all work just fine. About documentation you are right, Juice FX includes documentation for its scripting system but not for the main program, which is a miss on my end, but I don't recall having complaints about the lack of documentation or not being user friendly over this 5 years since the app was released, so I didn't look into it. This doesn't nullify your point at all.
In the future If you are interested in any of my other apps, I always include demos of them and I'd strongly suggest to check them out to see if they fit you before making the decision to purchase them. I have a "no questions asked" refund policy in any case tough.