No need to refer to their professional synths.
The YM2612 was a stripped down version of the PC-88/98's superior YM2608, and it just didn't hold a candle against the terrific YM2151 used in arcade machines and X680x0 computers. Yet, as with any soundchip (be it cheap or not), it's output quality really depended on the talent of the composer -- check Mega Turrican (Turrican 3) which is a great example of what could be done with this FM unit.