One of the differences in the Raspberry Pi 5 is the bootloader is in EEPROM. It has 2MiBytes which they doubt would be enough to fit a UEFI implementation. So it sounds to me like there isn’t much chance of UEFI this time.
2MiB could be enough space for an EFI implementation, but some features definitely would need to be disabled and it would definitely depend on how big the code for interfacing to the specific hardware on the board actually is. 4MiB is often big enough to have a functional EFI implementation, so 2MiB doesn't sound crazy to me, but I haven't tried.
