Getting "some things" to run is 0.001% of work of writing an emulator. Reaching "50% of the software corpus for the system" is 1% if the work, reaching "90% of the software corpus" is 10% of the work, reaching "99% of the software corpus" is 20% of the work, and so on...

The long tail is long, and for older, more esoteric systems, it is veeeeeeeeery long. You'd be surprised how far a 286 emulator will get with broken implementation of carry out sometimes.

Source: wrote more emulators than I can count