Yeah, that's exactly my experience. I built a framework for quickly prototyping with web components before I discovered the CSS-isolation defect. Sadly it's pretty much killed that idea as unworkable.

<https://codepen.io/webstrand/pen/jOzYVpL> is as far as I got. The FOUS is pretty annoying, too, before the templates get properly registered. But I could live with that.