Handling data React passes all data to Custom Elements in the form of HTML attributes. For primitive data this is fine, but the system breaks down when passing rich data, like objects or arrays. In these instances you end up with stringified values like some-attr="[object Object]" which can't actually be used.
Handling events Because React implements its own synthetic event system, it cannot listen for DOM events coming from Custom Elements without the use of a workaround. Developers will need to reference their Custom Elements using a ref and manually attach event listeners with addEventListener. This makes working with Custom Elements cumbersome.
Stencil provides wrapping to circumvent these limitations. https://stenciljs.com/docs/react
That's the selling point, but is not quite true. At least a while ago there was still some ceremony needed at least in React. You could say that that's React's fault, but it doesn't really matter whose fault it is - Web Components are different from standard DOM elements, and thus need special treatment from frameworks that interact with standard DOM elements.
The way that frameworks that work well with web components do it is be not giving special treatment to things. Vue, Angular, and lit-html all treat built-ins and custom elements then same, and they have different syntax for setting attributes and properties.
React treats elements different from components and sets properties on React components and attributes on HTML elements, with no syntax to set properties on elements. Then it special cases a list of attributes so that things like `class` map to `classList`.
So... remove special cases, add general abilities, and it all just works.
Whether React should or should not be doing that is irrelevant; what's relevant is that as a developer, you need to be aware that you can't "just" use Web Components in any framework.
You've correctly pointed out that the special cases are the problem, but are putting the blame on the things that are not special cases instead of the thing doing the special casing.
This is fundamentally React's problem. It's the only major framework to even have issues here.
React just had a more complicated way of setting properties. Entirely due to React's choices.
tl;dr you'll still be doing the wrapping in practice.
I wouldn't say that - Web Components (by themselves) do not allow you to write your view as a function of application state, which is the main draw of at least React.
But the initial use case of Web Components is to create a standard that allows developers to iterate on the web's built-in components (inputs, date pickers, etc.) without having to wait on the standards track, and FAST appears to intend to provide a number of such components.
(And there is more to such components than just styling (which is what Bootstrap provides) - it's behaviour, usability by people reliant on e.g. screen readers, an API to access its data, etc.