The only emulator that has serious problems is Daedalus (N64 Emulator). R13 can run some games fairly well, with most of the effort on SSBM and Mario Kart, but there are many games that still run very slowly (10fps) or not at all. Zelda: OoT for example, is said to "artifact". I don't know what that means, but everybody says its unplayable.
Everything else that I've ever tried/heard of works fine on PSP. Seriously, everything.