[jungler]

Comparing difficulty between FM and SID is a bit apples to oranges. The SID has analog variance in its filter, and no two are exactly the same, so a pure DSP implementation has to do rather heavy math to approximate the filter. As a result most people lean on a handful of SID emulation cores because otherwise their emulation is just wrong.

OTOH the things that tend to differentiate implementations of Yamaha's FM are the sample rates, bit depths, and envelopes used, plus any output distortion(YM2612 for example has a well known distortion in its implementation that adds a harsher edge). The core algorithms they use are something a high school student could pick up and do something with, and emulation quality issues are more a matter of it being easy to write an emulation that gets it 98% correct without covering the last steps, since those are details that really need error-for-error reproduction of the original ASICs and boards, including any timing issues - things which frustrate emulator writers everywhere.