I migrated yesterday and was very surprised to find that my old messages were not automatically migrated as well.
So after some going forth and back I managed to launch an import of my old messages BUT...
1- Sent messages were not migrated into the sent messages folder
2- Certain calendar items were simply gone
3- In the new client I can't invite people for a meeting.
This worked just fine in the old client.

What has been going wrong?