I am not too very happy that we need at least CSS/HTML/Javascript (ok, HTMX...) for web applications and would love to have a simpler tech stack.
For me, the biggest concern is CSS/HTML/JavaScript do not go away and it seems to me, when I choose FastHTML I still need a descent understanding of these AND need to understand how FastHTML transforms Python code on top of it. Templates show me mostly what I will get once they are rendered, which means less mental work for me.
Templating w/o embedded logic like Mustache are acceptable for me and I found good use cases for them. Once templating systems become obviously Turing Complete I see a problem. ;-)