Very well put. Its important to understand its not the physical hardware that is the problem, but rather the translation of the functions. Undocumented features are hard to emulate, because finding a way to translate that call/function to say, DirectX from the say, hoopy joopy SDK used on the saturn, can be near impossible, or incredibly frustrating since its a trial and error (read: fail and fail some more)
Which is important when understanding why new hardware is not emulated well or at all: documentation.
The GBA for example, came to light with alot of documentation behind it, and look at how well its emulated. The ps2 is much more complex, and all of its inner sekrat's are yet to be discovered =p