Here is a list of the chips on the saturn:

• 2x Hitachi SH-2 (main processor)
• 1x Hitachi SH-1 (CD-ROM controller)
• 1x Motorola 68EC000 (sound controller)
• 1x Yamaha YMF292 (a custom synthesizer chip)
• 1x VDP1 (custom graphics chip)
• 1x VDP2 (custom graphics chip)

The graphics system, the main processor, and the sound system have their own RAM. Additionally external RAM could be added as a card.

Basically all of these chips with the possible exception of the CD controller need to be emulated. Individually none of these are hard to emulate (although licensing might complicate that). Emulating all these chips plus the timing behaviors between them and their RAM is quite hard. Certain techniques can be used to make emulating the individual chips fast but many of these either break down or become significantly harder when you need them to interact with other systems with precise timing. Tight timing accuracy is likely important as some games for these platforms made a lot of assumptions about the hardware they would be running on and may also have exploited undefined or unexpected behavor on the system to increase performance.