The SC-55 only approximates the sounds from the stock MT-32 library by matching the instruments with its own GM equivalents. Not the custom timbres that a lot of games send through SysEx to the MT-32. They're completely customized sounds by the composer and there's no way the SC-55 can guess a custom timbre sound or even realize it exists in the game it's running with. So if you played QFG1 with an SC-55 it would sound horrible (all wrong instruments). However there are custom General MIDI drivers floating around the net somewhere that will work with QFG1 and other SCI0/SCI1.0 games that don't natively support GM.

It won't even play correctly with a game using stock MT-32 instruments unless you set something on the SC-55, I think. I don't know about that because I don't have an SC-55. I only have a CM-500 which doesn't have any controls outside of volume control.

not that I have used DOSbox that much, but can't you have GM via windows drivers from your soundcard ? Why get a seperate unit?

DOSBox will send MIDI commands on to Windows. The Windows default is a software synthesizer, though. It's not bad for most purposes, but it pales in comparison to a real Sound Canvas. The SC really does sound excellent. The MT-32 sounds great with software designed for it (and because of changes in the MIDI standard [well, the establishment of a MIDI standard, actually] most later modules sound dreadful when dealing with MIDI intended for the MT-32).