According to Jack Wallen who wrote the article "Create a portable version of Clonezilla live with Tuxboot" in 2012:
The problem with UNetbootin is that it doesn't support Clonezilla, one of the most popular open source, bare-metal recovery tools. If you want a USB version of Clonezilla, you can either go through the command line-based steps to get it installed (which is not for the faint of heart), or you can use a UNetbootin spin-off that supports Clonezilla called Tuxboot.