I'm currently using Unpoly for a new project and I love it so far.
I've tried HTMX in the past, and I liked it, but having to write custom endpoints for HTMX requests is annoying. Unpoly lets you write a regular backend, and it will detect itself which part of the response it needs from the server. If you need to optimize your responses later on and only return the necessary snippets of HTML, you can always do so.
What I don't like about Unpoly, is the website. The documentation is great, but their landing page looks like it's some framework that hasn't been updated since 2004. (Which isn't the case, v3 was released only this summer and has already received several updates and fixes).