One factor (but possibly not the only one) in some not working is the limited driver support available in the pre-boot loader environment. Smaller UPDs are likely to be older in design, and therefore compatible with older versions of the USB mass storage drivers.
Sky Player?? That has never been available as standard; there was a custom exploit which hooked into Humax's development work, but even that died a very long time ago. IIRC you also had to have (or obtain) a spare Sky subscription pass.
I presume that custom portal and portal-xtra1 will be removed from the Package Management >> Available screen before long as support for pre 1.03.xx Custom Firmware is being discontinued and only the new-portal is compatible with 1.03.xx