We just had to migrate over the weekend and we ran into the same issue with Robocopy and permissions on Unity. Somehow it did not flag us during testing. We tried EMCcopy too but for some reason it did not work. Someone recommended GS Richcopy 360 saying it worked with copying permissions to unity. We tested it and it worked just fine. This is how what we used for the rest of the migration (still ongoing).  It is also GUI based so easier to use. It was panic hours for us and I saw this thread so just wanted to make a point to update it.  I am not sure why robocopy would not copy permissions.  
Hi, we had mad a NAS server in EMC  Unity 380F & planing to migrate our Windows File server  to NAS server, i tried coping a test folder using Robocopy its copied but without sharing permission, is there any tool that we can use for file server migration. please advice.   Also can we publish NAS server directly from EMC unity, because once the server will migrate there will be difference name & need a server for DFS   Thanks in advance
Windows File server migration to Unity NAS Server