And then suddenly you need to be aware of the quirks: https://htmx.org/quirks/
And then you need your backend to handle all the weird new headers: https://htmx.org/reference/#request_headers
And then you need to learn completely non-HTML things like "attribute inheritance" https://htmx.org/docs/#inheritance
And then you need to learn a whole new DSL https://htmx.org/docs/#trigger-modifiers
HTMX probably has more concepts than react at this point in time.
I do think it is a bit overengineered however, though there are other implementations of the idea.