*[[PSG]] cannot be accessed directly as it is behind the PPI. It is simple to emulate though, but it cannot do much.
*[[FDC]] is a black box internally, while the C64 uses a regular 6502 as FDC.
Also, on the software side, the CPC has 2 main operating systems (AMSDOS and CP/M). While the MSX has only one (MSX-DOS) but that is binary compatible with CP/M.
Thankfully, many tools and tests exist to help emulation authors.