Wonderfully vague on details. Two of the harder issues to emulate are the weird floating point and the gpu. Floating point on the PS2 is slightly different from ieee754 (no infinite, different rounding), and emulating it using a standard Intel/AMD cpu is extremely slow. (I have an idea to speed it up using doubles, and I'm curious how Sony has solved this). The SPU's on the PS3 have the same weird floating point, so this made emulation easier.

One other problem is the slightly unique gpu in the PS2, called the Graphic Synthesizer (GS). The gpu in the PS3 was not able to emulate it, just because it is so different. (More specifically, the GS has 32 bit z index, and scissoring is different.) Sony had to include an actual GS in the PS3 for PS2 emulation. They took this out to make it cheaper, and this is when they lost PS2 emulation. I believe modern GPU's are flexible enough to emulate the GS, especially since Sony has access to the complete GPU.

SSX, the original and "Tricky" titles! I'm off to find old PS2 disks now.