The major downside of using light DOM is that elements cannot compose neatly since there's no delineating between what the component itself rendered and child content.

When you need to re-render your component, how does the component know not to replace the child content you rendered, Vs the child content it rendered into the light DOM. That's why the shadow DOM and slots were necessary, because then there's no intermingling of your content Vs the component's

This may not be a problem if you don't intend to compose your components. But if you do you will hit limits quickly