I have spent a few months this year trying to wrap my head around FM synthesis. Unfortunately I don’t think the DX7 is hard to program just because of the user interface. I have read up and thoroughly understand the theory behind FM synthesis. I have followed various tutorials to design patches. At the end of the the day FM synthesis is powerful but it is inherently much less intuitive and also much easier to get horrible ugly sounds out of than something like the subtractive synthesis seen on most analogue synths.

The reason FM synths need to be digital is that very precise pitch ratios between oscillators are important in order to get good sounds. For example you may want to modulate the pitch of one oscillator with another exactly a fifth up (3 to 2 frequency ratio). It can easily sound terrible if the fifth up oscillator goes slightly out of tune.

These sort of precision and nonlinearity in output are also one thing that makes FM synthesis a lot harder to use.