I believe there are a lot of people on this site who would disagree with that opinion. God knows I do.
It may not have been sample-heavy like the SNES sound was (which I know a lot of people loved the SNES for), but I believe the Genesis still put out fantastic synth music over the years... both in composition, and in sound. The likes of Thunder Force IV, Streets of Rage and Master of Monsters (among many others) show this quite clearly I find. In fact, there are some soundtracks that I feel came off better on the Genesis than the SNES, such as Earthworm Jim, and TMNT: The Hyperstone Heist (which has the same soundtrack as TMNT IV). The synths of the Genesis give them a different, harder feel, and there's no sample cut off (which was present in a good number of SNES titles... and which bugged me a lot).
However, it all comes down to opinion, and it looks like ours are at odds. Some like the FM synth sound, some don't. But since you gave your thoughts voice, I thought I'd do the same for mine.