To be brief: When using Synthesia with my workstation keyboard (which naturally has hundreds of sounds), it doesn't output the piano sound that I necessarily want.
For example, it will play the sound assigned to "Acoustic Grand Piano" on my keyboard, but in fact, I want the sound "Studio Grand Piano" (which is the default anyway). This is mostly personal preference, but I've also noticed some weird distortion issues mostly when previewing songs. I have a Casio wk7600.