The trouble is the internal bandwidth of the GS. It's just shy of double of the PS3s total internal bandwidth.
There are ways around the issue. But the emu would basically need to predict what the game is trying to do and use the PS3 in other ways to do the same.
Is that really such a dealbreaker, though?
PCSX2 seems to manage to emulate a lot of effects without the 'massive' GS bandwidth. At worst the PS3 could skip some effects, or render than at half fps, which would be ok if they are not too extensive.