I had a whole slew of ScummVMX ROMs that simply refused to FTP.
I mention this because I've been trying to sync my two most used Xboxes, mainly for backup purposes and have run into those 'quirks' particularly when transferring certain ROMsets eg. However UnleashX has certain unrecorded 'quirks' which can still cause you problems.
When it works it is easily the most convenient and a quick way to get stuff from one Xbox to another. I know this is an old thread but I've had quite a lot of experience FTPing directly between two Xboxes using a cross-over cable and the built in UnleashX FTP client.
