Lots of things are changing. Here is Google's migration guide.[1] I've also personally seen some issues with the `<all_urls>` and `https://*/*` permissions to allow access to all pages, though I have not seen those well-documented elsewhere and it could just be user error. I've gone back to Mv2 for now, planning to fork Chromium to re-enable it for my personal extensions.

https://developer.chrome.com/docs/extensions/mv3/intro/mv3-m...