theirs probably a full on blog/forum post on some site somewhere that goes into almost insane detail on why official emulators are pants, usually it has to do with slowdown and glitches
N64 is ment to be the hardest machine to emulate
The problem is a lot of people that make hacks of N64 games only make them for emulators, whats the bloody point of making a ROM hack that wont run on original hardware?
Are there any decent Jaguar ones knocking about?
Yeah many emulators (including official) are clearly not 100% accurate, even on reasonably powerful host hardware. I think the bsnes/higan one emulating the Super Nintendo/SFC is mean't to be 100% compatible and most crucially accurate, and requires a fairly powerful CPU to achieve it (so absolutely no chance of 100% cycle exact accuracy on a Wii or original Xbox). Its only available on PC's though from what I gather. I tend to just use SNES9X and its various console/android ports as it is pretty competent (and I steer clear from the very old and inaccurate ZSNES).
N64 emulation does appear to be really challenging, so the Everdrive 64 was the first flashcart I ever got, as it makes perfect sense. The other cartridge everdrives are not so compelling as emulation is pretty decent for 16 bit machines and prior (and can be run more than acceptably on lower spec host consoles). I do have an Everdrive GB, but lets face it, even a Nintendo DS with its puny 60mhz ARM processor can emulate original GB games very proficiently (although i've noted some glitches on GBC games on DS!)
As for the Jaguar, i've used Virtual Jaguar on PC with mixed results, on a reasonably powerful i7 laptop. Seems a but unoptimised, as I guess the desire to play Jag games isn't that high, plus it may be a bit of a complex system to emulate. Maybe our resident Atari man Laird can suggest some!