I just hate the fact that it requires JavaScript to render even the simplest of pages. I would be bold enough to state that most websites built using React have absolutely no need to be built with React, i.e. they should be using plain HTML/CSS and maybe a tiny bit of JS instead.
Instead we now have an entire ecosystem where JavaScript powers everything, even the rendering of the most basic static page. And have you seen the markup generated by React and its ilk? It's horrific and makes debugging/modifying pages using client-side tools (user scripts, accessibility tools etc.) much harder.
It's anathema to me. I hate it all.