Liberté had to be installed on a separate partition, which had to be the second one (Windows sees only the first partition on removable disks). I don't think that there is a relatively easy way to accomplish such an installation from Windows.
Syslinux, on the other hand, allows the distribution to be a simple directory on FAT/FAT32-formatted disk, and provides installers for both Linux and Windows the installer just modifies the boot sector to load ldlinux.sys, and possibly also modifies the MBR if the installation is to a partition.
I think that UNetbootin (which I understand is essentially a wrapper around ISOs) is an unnecessary layer that complicates things.
By the way, I think that for LiveCDs (as opposed to LiveUSBs), GRUB and Syslinux are equally simple to install and to use, but I only tried GRUB LiveCD support was removed from Liberté Linux before the transition to Syslinux.