Most of the complexities lie in managing the various configurations of total system compatibility emulation, especially for timing, analog oddities, and whether

Most of the complexities lie in managing the various configurations of total system compatibility emulation, especially for timing, analog oddities, and whether to include bugs or not and for which steppings. If you want precise and accurate emulation, you have to have real hardware to validate behavior against. Then there are the cases of what not to emulate and offering better-than-original alternatives.