Seems like the main concern was not being able to extract the editor component to use in other browsers. This could have been solved with the Shadow DOM polyfil

Seems like the main concern was not being able to extract the editor component to use in other browsers. This could have been solved with the Shadow DOM polyfill, but it has a performance cost to it. Polymer's faster Shady DOM shim wasn't extracted into a standalone library until recently. The issues with themes and shared styles are largely solved by CSS Custom Variables, which inherit through shadow boundaries (much like /deep/), and Shadow DOM v1 (the new spec implemented by Safari and Chrome) which allows <link rel=stylesheet> tags inside shadow roots.