Originally Posted by
flamepanther
Dreamcast and the last several generations of Nintendo consoles use OpenGL or a similar 3D API that can be easily translated into OpenGL. That means you can more or less handle the graphics side of things natively instead of actually emulating the graphics hardware. Then on the CPU side, they tend to have a simple setup based on standard, well-understood hardware. PS2 was a Gordian knot of multiple custom chipsets. It is poorly understood, through reverse-engineering only, and is therefore a nightmare to emulate, despite being considerably less powerful than Nintendo systems that are emulated quite well. Saturn emulation has faced similar difficulties for the same reasons, despite being even less powerful than that.
As for XBOX, I have no idea why that's so tough, since it was all just tweaked PC hardware running a modified Windows kernel. I suspect it's some insane copy protection scheme standing in the way.