Developer usability: the fact that they're largely analogous is literally why the implementations that stick to one language, taking advantage of new language features as they pop up, and abstract away all the bindings, win out over something like Polymer, which sticks to the separation of HTML, CSS, and JS.
React, for instance, has changed a lot since its first release, mostly to do what devs need it to do with a bare minimum of coding on their part. That is in stark contrast to Polymer, which requires devs to write out a lot of the logic that you simply get "for free" with React. Writing a pure JS class that "just works" just because it has the two words "extends Component" is a hugely winning benefit, for example.
Take something like https://codelabs.developers.google.com/codelabs/polymer-firs... - it certainly does the job, the code is clean, and the separation of concern follows the traditional HTML/CSS/JS model. But as a developer, what I see is an inefficient implementation that requires me to put loads of stuff in that should be taken care of for me: "Why would you need to say what my component 'is'? Tooling should see that already.". Also, all browsers already handle arbitrary properties, so "Why would you need say which properties can be used, except for schema enforcement". (if I say then every browser already sets that attribute key:value correctly). And of course: "Why would you need to specify which events to listen to instead of just implementing the relevant function hook? Tooling should take care of the rest".
So what I _want_ to write as a developer who's used to efficient web-component-like libraries is something like:
import { register, Component } from 'polymer'
register('icon-toggle', class IconToggle extends Component {
onAttributeChange(attr) {
if (attr.name === 'toggleIcon') { ... }
}
onPressed(evt) {
this.pressed = !this.pressed;
}
});
And we're done, that should tell the library everything it needs to know.
It really is just a matter of "what lets me do the most while saying the least". Polymer requires me to write a book where React or Vue let me write a memo. Does it make it harder for a search engine to index my site? Sure. But that's what seo/og is for. Does it make it harder for a user to inspect-element? Sure, but that's their problem, not mine. Does it make it harder for someone else to learn how I did something if I don't host the code on github or something? Sure, but I am under no obligation to write code that, in production, is easy to debug and replicate.
So while Polymer is super strong for all of those, realistically: I only care at the academic level, because that's not where my time and efforts lie.