I’d like to answer this. Syncing reminders and calendars seems to work across operating systems regardless of version. The syncing is done using iCloud, and as someone who is running the beta on iPhone and iPad while still using the previous macOS, I have not had any issues with syncing.
As for fantastical, it works, but the reminders portion can not be edited using fantastical. If you attempt to change the default reminder list in fantastical, the app crashes. If you attempt to show reminders in the fantastical widget, it often won’t display anything. I will be pleasantly surprised if the PB2 fixes this problem. Other than this, fantastical still works on iOS 13