Our Endomondo support is in public beta! If you notice anything weird, let us know.
We get notified whenever add a new activity in Endomondo, and thus, the sync button usually does not find anything. If you are missing something and you are a Pro user, you can use our Historic Sync feature or contact us.
Yes, by toggling the Prefer Private Activities on your Settings, we will automatically mark activities uploaded to Endomondo as Private.