It shouldn't be. The hardware and software development toolsets used by modern game systems get ever-so-closer to commodity PC hardware with every market cycle. Older hardware is actually harder to emulate than newer. NES cartridges had their memory controllers built into the cartridge. The newer ones used by games such as Castlevania 3 are imperfectly understood and many games still have crash glitches. Only roughly half of these controllers were made by Nintendo and not all of the ones that weren't were licensed. The situation with the SNES is in some ways better and some ways worse.