In my experience over the years, FM synthesis is a vastly inferior music hardware technology compared to the MT-32 and Sound Canvas that were available back then. The only exception to this (in my opinion) is Japanese games. Most consoles and arcade machines used FM synthesis chips, and somehow the Japanese got really good at making kick-ass soundtracks using them. Most PC games released in the United States were by American or European software companies, however, so their music sounded better on higher-end MIDI synthesizers.
DX200 6 operators:
FS1R 8 operators:
Not what I would call "vastly inferior"
6 operator FM has been available since 1983 in the DX-7 (it was the first FM synthesizer Yamaha sold)
The OPL3 in 4 op mode can actually make some pretty good sounds (synthy sounding of course), nobody ever actually used in in 4 operator mode in a game as far as I know. They'd just use it as an OPL2 with more channels. The ym2151 was very popular in arcade games and could also do some good synth sounds. Yamaha based a few DX series keyboards on the 2151.
I wonder if they were using those to do the soundtracks for ym2151 based arcade games?
In my opinion GM was a bad, bad BAD idea, the use of ROM sounds is where PC audio went horribly wrong. The GUS is what the SB-16 should have been (but maybe on a lesser scale, e.g. 8 channel ADPCM to avoid the need for memory on the card)