Aside from poorly documented hardware, one reason why Xbox emulation is in such an early state was simply lack of interest. The Xbox had a meager first-party library and what exclusives there were, were already available to play on every Xbox released ever since via back compat.
I think a more tragic case would be if MGS4 was ever re-released as part of the Master Collection ports. That game was designed from the ground up for PS3, and runs terrible even on the custom RPCS3 builds designed specifically for it.