Just awful. 10 years in the making (if you can call endless bikeshedding committee meetings the "making" of anything) and still barely usable.

It offers nothing in way of ensuring that custom elements behave like builtin HTML elements. Half the elements I've come across will break or perform no-ops when you update an attribute or set a propety after it was attached to the DOM. Nevermind detaching and reattaching to the DOM, which will break virtually all of them (including my own sad contributions to this space).

The exception to this are those built using a 3rd party library like lit-element or stenciljs, which fill in the obvious omissions of these specs. Perhaps in another 10 years, a mangled version of half of one of them can be standardized? In the meantime, each component shipping its own frontend library or inlining the same core functions over and over again does nothing in the way of interoperability. You can bundle every popular JS framework and mix their components today. The reason you don't do it then or now is bloat, not the lack of a minimally viable shared component interface.

Besides, if you're going to use a 3rd party library and associated bundler/compiler, you might as well pick a good one such as React, Vue, Svelete, Solid or even jQuery UI. Using any of these, you can design and build an entire app faster than the bikeshedding commissars from goog and aapl can agree on whether "open" or "closed" should be the default for attaching a shadow DOM (at the risk of ruining the joke: there was no agreement; the developer has to provide a value in each instance...)

There's also a lack of standard server-side-rendering support. From last I looked at it, some of the web component libraries have their own SSR support, but that only works with web components authored with that specific library, negating the supposed universal compatibility of web components, and they involve using heavyweight simulated DOM libraries on the server. I'm not going to bother with something that doesn't take SSR seriously when React aces that so well (especially with the newer server components support) among other things (like giving you a code model that ensures by default that updating a prop at runtime causes the same result as setting it on creation).