Actually, I'm going to have to agree that the system's sound hardware was pretty damn limited.
The biggest problem, in my mind, is that the PSG has absolutely no duty cycling. The NES can vary its two square waves to four different settings (although two of them are inversions of each other and sound nigh-identical, so it's really more like three); hence, it's why the instruments in
Super Mario Bros. 2's music sound different from the instruments in the first
Super Mario Bros.'s music. The Master System, lacking that, is stuck with a bog-standard square wave, so every game's music uses very similar-sounding instruments, as such.
That said, there's still a fair bit of variation there, even within my limited frame of reference (the
Sonic games,
Land of Illusion starring Mickey Mouse and
Baku Baku).
Underground Zone opens with something that sounds kind of like big brass.
Gigapolis Zone makes me think more of some kind of jazzy piano (rhodes, maybe?).
Meta Junglira evokes steel drums and exotic-sounding woodwind instruments with quite a bit of style.
Tails Adventures's map theme similarly pulls off steel drums really well, making a fairly inviting tropical island theme (a bit jarring compared to the opening stage, which is basically
a forest on friggin' fire).
Sunset Park Act 3 is honestly one of the best tracks in the entire
Sonic series. So, yes, it
can sound a bit samey, but even then, you've still got standouts.
Also, the Master System supports FM synthesis;
compare "Magical Sound Shower", for instance. Shame that never really expanded beyond Japan!