I'm a little worried this is a dumb issue that I'm running into but here goes:
I just created an ExtReact app following the tutorial here. Without changing any code I'm seeing the following error in Edge:
The app loads fine in Firefox and Chrome but not Edge or IE. Obviously it has to do with custom elements but I don't know what else I'm missing or if I've misconfigured something.
You need a webcomponents polyfill