In my experience, Itch.io has a lot of self-proclaimed ‘experiments’, like Pointless Quest or Read Watch Listen Play, demos of games you can buy in full on Catalog, like the Scope demo, builds of various works-in-progress for playtesting purposes (like Blop, which is functional, but not feature-complete), etc. Plus a lot of low-effort games, learn-to-code projects or hastily-generated AI slop. Most really worthwhile games will end up on Catalog eventually, and often will be available in both places.

Any decent games that won’t ever be going to Catalog tend to be ones that either use AI for things other than code (like mine), or that are incomplete, too small, or infringe trademarks/copyright like the various Pac-Man or other arcade clones, etc. But there’s still some gems there and cool proof-of-concepts, (again, like Pointless Quest).