Performing the instruction is a 90% solution. You do a 90% solution for the CPU, a 90% solution for the PPU, a 90% solution for the sound, etc, and implement th

Performing the instruction is a 90% solution. You do a 90% solution for the CPU, a 90% solution for the PPU, a 90% solution for the sound, etc, and implement the dozen or so most common backswitching chips. Then 75% of games are playable and 40% of all games are bug free. Maybe less. Then you start working on those last 10%s, which includes cycle accurate emulation. As it happens, there's a lot of work with goes into the final 10% of the various NES subsystems. Far, far more work than the first 90%. As for what percentage of games need cycle accurate 6502 emulation I couldn't tell you.