React makes it fairly easy to implement your app in a way that makes it easily portable to different platforms. With things like HTMX, you'll more or less have to rewrite everything if you ever end up in a situation where you'd like to release the application on mobile, or as a desktop app, especially if you want it to work when you can't rely on the user having a stable internet connection.
I have used HTMX with Flask and Jinja. It makes the process much simpler to do the frontend development as a backend developer. But I can see its limitations. It's not suitable for anything bigger than a hobby project. Also, it doesn't help with keeping the frontend and the api totally separate. You have to return the html object from API which has its own set of problems.
