Reflex converts the Python code into a react frontend and fastapi backend. It's an abstraction that hides much of the underlying foundation, which is a very differently philosophy to FastHTML -- which endeavors to make the underlying foundation directly available.

Personally I wanted to create something that made the foundations of the web more directly available to Python programmers, rather than hiding it behind multiple layers of abstraction. Reflex is very impressive though, and I expect for some types of app it might be a better choice; probably worth trying out both!