I believe you will find two main sources of differences in tone between the various DX7 emulators. One is that there are fairly major differences between the original DX7 and the DX7 II (I used the latter for the original engine now adapted in Dexed). The other is the analog filter on output, approximately a 16kHz lowpass filter designed to reduce artifacts from the DAC (this is replaced by a more general Moog-style filter in the Android implementation but present as an accurate emulation in Dexed).

I think Dexed is quite accurate, but this work will allow the authors to take it to the next level. I suspect most people won't be able to hear the difference, however.

Nothing to add but I wanted to say thank you for making Dexed, we've used it in a few synthwave-esque songs.

This is what we were aiming for so I'm super happy to see confirmation of that :) Ken is a true wizard.

Strangely, I haven't used any of the emulators or a DX7, so I can't comment on the differences in tone.

Ha, that would have been my question as well...I've often come across the idea that digital synth emulations are necessarily "perfect" and indistinguishable from the original, but is that really true in your opinion, from a first principles standpoint? If not, which component would make the emulation most difficult?

If you look at it from that perspective, there are two factors. First, are the digital values identical? This is something my research can help with, using exactly the same exponential values (bit width, rounding, etc.) Second, a digital synth produces an analog output, so even if the digital values are perfect, the digital-to-analog conversion is going to produce its own distortion, filtering, etc, which can be pretty substantial.

Also see raphlinus's answer.

I'd like to add to the list Korg's own FM implementation MOD-7