I don't know which version of UBCD you are using. I also don't know which version of Syslinux you are using.
Whichever the versions, memdisk requires RAM for the ISO image. If the ISO image you are trying to load is "too big" for the RAM available, it might probably fail.
Another detail might probably be the "raw" parameter you are using in the append line. Why you think you need it?
An alternative method to memdisk, would be to expand each ISO image and use ISOLINUX/SYSLINUX directly. I won't get into the technical details. You might be interested in using auxiliary tools such as YUMI, SARDUCD, LILI, or some of their alternatives under Linux.
Whichever the versions, memdisk requires RAM for the ISO image. If the ISO image you are trying to load is "too big" for the RAM available, it might probably fail.
Another detail might probably be the "raw" parameter you are using in the append line. Why you think you need it?
An alternative method to memdisk, would be to expand each ISO image and use ISOLINUX/SYSLINUX directly. I won't get into the technical details. You might be interested in using auxiliary tools such as YUMI, SARDUCD, LILI, or some of their alternatives under Linux.