> Or is this not actually the case?

According to a conversation I had with someone, I think it was _Demo_ (but don't quote me on that), someone had managed to write a video plugin with basic low-level emulation of the RDP, enough to get Super Mario 64 displaying things with "proper" emulation, but it was very slow (I can't understand why) and had spurious graphics glitches, as the person I talked to estimated that currently, non-HLE RSP emulation is about 95% accurate.

I'd say the main problem with getting accurate N64 emulation would be dumping the internal ROM in the PI that it initially boots from.