> The library is very useful for (?) UI problems...
Components are good. But they didn't come from React and React isn't the only place to get them. Angular 1/2, Polymer, Ember are great in comparison. And so much more simple. React's position on simplicity is that the demos on the site are all 15 lines so that means it's simple right? You can't say it's complicated because it's 15 lines, see? It's "easy to reason about", i.e., "you're not smart if you don't get it".
Once you see the toolchain and ecosystem you will glimpse the price of those 15 lines.
> ...toolchain fetishism which tends to cloud React's fundamental simplicity
This is a very real thing. People who work in React spend way more time fixing toolchain stuff and trying to mix 10 artisanal-crafted "easy to reason about" libraries in order to accomplish tasks which are either solved or not even issues in other component frameworks.