I'm not sure it's really true that the NES represents a single hardware target. There's two families of CPUs (2A03, 2A07) with different clock speeds depending on whether the device is NTSC or PAL, within each family there's a half dozen or more revisions, DRAM controller chips changed frequently causing many interesting variations in how and when the object attribute memory could be read (or not read, as the case may be).

And that's just the NES devices! The Famicoms were different again, there were at least two licensed clones, and dozens of unlicensed clones (if you, eg, wanted your game to sell in Russia, you'd care about being compatible with the Dendy as well as the genuine NES).