It's true that React doesn't let you build web components. React does not use or polyfill Custom Elements or Shadow DOM. It's also true that React is "monolith

It's true that React doesn't let you build web components. React does not use or polyfill Custom Elements or Shadow DOM. It's also true that React is "monolithic" in the way that other frameworks are: they don't naturally interoperate, or do by cumbersome adapter libraries that require wrapping widgets and marshaling between data-binding techniques. They interfere with normal DOM usage, and tend to break if other code modifies a component's DOM (granted Shady DOM has made this trade off temporarily), and they aren't "just elements" - getting a node controlled by React out of the DOM doesn't get you the component itself, no useful API is added to the node. You simply don't see many applications that mix and match React, Angular and Ember because of this. This Polymer difference is that it's deigned to help you create "just elements" so that they're interoperable with other frameworks, plain JavaScript or just HTML. Users of Polymer elements should never have to know that they were made with Polymer. Needing to use Shady DOM APIs is an unfortunate exception to this, but an site can choose to use the full Shadow DOM polyfill if the performance fits their needs.