Sorry for the double post, I can't edit my last post.
Compared to more proprietary consoles such as the Sega Dreamcast, Gamecube, or N64 the XBOX 1 is probable easier since a majority of it's internal workings are very well documented withing the PC industry and thus require little reverse engineering.
No, actually the fact that Xbox is PC identical makes it much harder! Is a PC easy to emulate? Nope. It's a common assumption that Xbox is easy to emulate because it's like a PC. It's not, and I'll explain why...
1. The x86 CPU is NOT easy to emulate. Clocked at 733MHz that would require serious horsepower to emulate it at full speed (in software). Take a look at GameCube's CPU @400+MHz. You'd be lucky to get 30 fps in a GameCube emulator. Sure the x86 architecture is well documented, but if you look at the register sets for each instruction, you'll notice that it's MASSIVE compared to most CPUs. For instance, you might think there's only one MOV instruction, right? Well, actually there are about 30+ different versions of the MOV instruction. The same goes with others such as CALL, ADD, SUB, JMP, INT, and the list goes on. Another thing about it is the MMU. Any experienced emu author knows that's a bitch to emulate. The worst thing is the fact that it's an out-of-order execution CPU. So that means it's impossible to create a cycle accurate x86 CPU emulator.
2. NVIDIA GPUs are hard to emulate. They are very poorly documented and the GeForce3 is one of the most poorly documented of GPUs. Even a RIVA 128 is hard. To this day, I have yet to see anyone actually emulate an NVIDIA GPU of any kind! The register set of a GeForce GPU is much larger than that of many CPUs. Thanks to openxdkman, the NV2A is finally documented enough to be emulated, but not fully as the list of GPU registers is far from complete and fully documented.
3. NForce audio (afaik) has NEVER been documented! The NVIDIA SoundStorm APU (afaik) doesn't even have open source drivers (from both professionals and amatuers). If they do, please let me know
The AC'97 is a different story, that's fully documented as well as the Motorola DSP 56xxx.
4. I'm not 100% sure if this is a problem, but since the Xbox has been known to have multiple different video encoders per version, emulation of the BIOS may have specific ties to a specific video encoder.
5. There are also other chipsets that aren't exactly PC identical. Xbox-Linux has helped document alot of things in this area, but not 100%.
I hope this clears the whole "Xbox is easy to emulate thing"