Wanted to post the known important issues so far:

Invalid aliases causes an internal error. https://git.cloudron.io/cloudron/box/issues/533 External domains are not migrated properly. When an app used an external domain in 1.x, this was moved as a "manual" domain in 2.x. If you try to enable email for this migrated domain, you will hit https://git.cloudron.io/cloudron/box/issues/525. Workaround is in the bug report.