At the bottom end they're really, really simple if someone else has done the grunt work of writing the CPU code. I remember back in the mid-90s my buddy and I wrote a Sega Game Gear emulator in one evening after grabbing a .c file for the CPU. Those 8-bit console GPUs are pretty simple.
But... once you get past the 16-bit consoles it all goes to shit. Something like a Sega Saturn with two CPUs and 3D hardware. Lord help me. I honestly would not want to even begin trying to emulate a PS3. Total respect to those guys writing emulators for modern consoles and actually getting them to play the software.