Of course it's always possible. But when you reverse rare arcade hardware, generally you only have to make something like 10 games working. On a console there are hundred games, if you don't have all infos, you approximate some stuffs, you check it with your games, but it's very rare that there's no game programmed in an odd way that has some problem because of your approximation. The RSP lle informations are undocumented and has been reversed by zilmar. It works for audio tasks in most ucodes. But there are still some rare ucodes used in some games that do not work correctly with it.
Don't think there's no approximation in MAME, just by reading aaron's page, you can see there are a lot. It's hard to transpose what have been achieved on one particular hardware to another.
Btw, are they using a dynarec for this r5000 core ? With a dynarec it shouldn't be that much a problem. If they do not want to use dynarec for portability reasons, they can at least implement some kind of opcode cache like i did for mupen64 core, it's much faster, don't have more compatibility problem than the interpreter and is still portable. Maybe they have already implemented such thing ? I'm asking because i'm curious and you seem to follow mame developpemnt closely so maybe you know