I've used HTMX on a personal project of mine. Other than HTMX itself, it used Go templates + Tailwind for CSS. As a backend dev with almost no professional frontend experience, I was able to replicate a fairly large and feature rich React app (which I was inspired by) using htmx in a matter of weeks in free time.

The main problem for me was storing/passing state between too many fragments. At some point some pages can become too complex to be manageable by HTMX, unfortunately. Lots of little fragments depending on each other, I began struggling to maintain a clear mental map of what was going on.

I'd say if React is more like functional programming, HTMX sometimes feels like GOTO programming.