I imagine there are a lot of ways a Migration installation on Win 8 could fail, but just to share my fix…
I did a Windows 8 Upgrade from Windows 7, and about 30% into the Migration phase, this error:
Setup has failed to apply migration data.
… Followed by a rollback to Windows 7.
This error tells you nothing – very frustrating. Look in the logs located in C:$WINDOWS.~BTSources (surf around in there and you’ll find the log file that applies, possibly in the Migration folder).
In my case, I noticed an entry that pointed to a Migration Error – it couldn’t move my desktop.ini file in my Music folder. It was looking for C:UsersbpotterMusicdesktop.ini and couldn’t find it, failed the migration, and triggered a rollback.
I deleted the desktop.ini file (I don’t really care if I lose the folder view settings for my Music folder) and the install and migration completed successfully.