I developed my game Mystic Mine on Kubuntu using nothing but Open Source tools: python/pygame, gVim, blender, the gimp, inkscape and audacity. I had no trouble getting it to work on any Linux (32-64bit), Windows and Mac OS X intel and ppc (installer packages included). Sound, graphics, ... no problem! I honestly don't know what they are complaining about, but they are probably doing something wrong or using the wrong tools.