The "dd" method you describe can be used with isohybrid images. UBCD ISO images are not isohybrid.
Booting with SYSLINUX, the boot media for UBCD is expected to be formatted as FAT. The "dd" method generates a boot media that will use ISO9660, not FAT.
Since I always use SYSLINUX as bootloader for UBCD USB drives, I don't even know whether (all) the tools included in UBCD (specially those in fdubcd) would work correctly.
Booting with SYSLINUX, the boot media for UBCD is expected to be formatted as FAT. The "dd" method generates a boot media that will use ISO9660, not FAT.
Since I always use SYSLINUX as bootloader for UBCD USB drives, I don't even know whether (all) the tools included in UBCD (specially those in fdubcd) would work correctly.