Checklist for Web Components

Web component's JavaScript exists in the global scope so it can overwrite and be overwritten by scripts external to the component.

This is actually a major issue in my opinion. You can isolate your code to some extent using an IIFE but if you want to include 3rd party libraries that quickly becomes very hard to do. 3rd party libraries will also often directly modify the window object making any isolation attempts ineffective.

<rant>No, it's cool, let's just keep integrating software components by one-off, non-reusable effort of reading documentation and wiring everything up manually.</rant>