I recently started moving data from my 6a to my 9a, and found a HUGE bug that could be really serious.
I used seedvault to move data from my 6a at first, moving my call history and messages. This became an issue and corrupted how messages worked. Other profiles could not get messages, but could send them. Typing in a number would send me to a different number in a convo, and some convos were linked to the wrong number.
This also happened (after a factory reset) when importing a contact export. Speifically it moved a convo I was having at the time with my partner to a family members number.