I don't know much about the import function, but as far as exporting from Reunion to Ancestry, this is what I do:
I actually use Reunion for all of my research and then save it to a gedcom file. I then trash what is on ancestry and upload my entire "new" gedcom file.
There are 2 problems that I ran into with this method:
1) any of the census, marriage, death, etc. records that were assigned to an individual are no longer attached.
2) notes that I have made in Reunion are not transferred to Ancestry.
I understand problem number 1. The program is unable to retain the attachments, because it is a new gedcom and starts over from scratch.
Problem number 2 however, is beyond me. I cannot understand why my notes are not transferred.
Don't know if any of this helps with your question, but I hope so.