For instance. "My system handles B-Major much better than D-Minor."
Do you know how hard it is to come up with something that gets ZERO Goolag hits? That is some "for instance"! No one ever, ever said anything even remotely like this.
For the very simple reason that what key the music is in is of a completely different order than what instrument is playing the music, regardless of key. It is the challenges of as frogman said, faithfully reproducing the sounds of the individual instruments in all their timbral and dynamic complexity that our systems do poor or well. If your system faithfully sounds like a real live guitar then it will faithfully sound like a real live guitar regardless of what key is being played.
And yes, I grew up being taught to play piano and accordion in grade school, and played french horn all through jr high and high school. Along the way learned to play trumpet, and even a little sax. Later on learned harmonica. The main benefit being what frogman said, a lot of first hand experience with the way real musical instruments really sound. He describes it very well indeed.