There is no real x over when it comes to the architecture of the two synths other than the digital subtractive filter plastered on top of the sound gereation section of the relative synths, and the fact that they are digital synths with digital chorus, delays and reverbs.
The static nature of the sampler / wave rompler / linear DCOs or whatever they were called ( compared to the FM operator architecture makes them sit world’s apart. The D50 doesnt come close to the contancerous, metalic side of FM.
And Roland tryed to combat the expesiveness of the DX (competition was the DX7) with the architecture of the synth having attack PCM samples and then seperate sustain “waves” to get movement and expression.
Because they were competing against the DX7 there are some Roland patches in the Ds and in JVs that do recreate great fm sounding patches, Especially some of the pads, with your eyes closed you would think they are FM. They did build a complex synth capable of some lush, complex sounds.
And the DN would never get close to some of those strings, choir voices in the D50 no matter how black your FM programing art is mastered.