I use one of the Betas as source, can this be the problem?
I use RC1, and 'BSOD'ed to start with. The names of some Files and Folders in boot.wim and install.wim actually contain version numbers, and the script for copying the files to the Target also uses file- and folder-names containing the RTM-version numbers. (... I'm not sure why wild-cards aren't used.)
The work-around, is to select the 'build: file-copy' script, and choose "full" file-copy. Then the entire boot.wim (using wild-card '*.*') is copied to the Target, rather than the selective files copied by "standard" file-copy.