The migration guide [1] lists breaking changes. If you use many Vue 2 libraries you'll want to wait until they're updated to Vue 3. (I reminds me of the Python situation where if you used lots of Python 2 libraries you'd write your new code in Python 2 instead of Python 3, until those libraries migrated to Python 3)
[1]: https://v3.vuejs.org/guide/migration/introduction.html#break...