Polyfills also add an interesting twist. You can use them to enable Web Components in Firefox, but they mimic shadowdom behavior imperfectly. You need to be very careful of how you use CSS in that case.
Lastly, there seemed to be some gotchas in ES6 module loading a few months back. Perhaps those have been fixed. At the time, I found it hard to debug modules that failed to load.
In any case, I'm always happy to see Web Components and more powerful/useful features being added to JavaScript in the browser. Frameworks like React serve useful purposes, and it's great that we have a platform that allows us to move quickly and bring new patterns to life without waiting for browsers. In the long term, though, I will always be an advocate for standards and making vanilla implementations feasible.