The PPUs use static logic and an external 21MHz oscillator (which also powers the CPU), so they're perfect for single-stepping in isolation from the rest of the SNES.
The PPUs themselves output only analog RGB, there's an analog pin for each color channel rather than a digital pin for each color bit.
On some level, I don't doubt it's possible to build a test harness that can automate things based on the analog RGB values with some fuzzy matching, but if we are going that far, it just makes more sense to snoop the bus traffic directly. That would reveal a lot more information and in digital form.
The PPUs themselves output only analog RGB, there's an analog pin for each color channel rather than a digital pin for each color bit.
On some level, I don't doubt it's possible to build a test harness that can automate things based on the analog RGB values with some fuzzy matching, but if we are going that far, it just makes more sense to snoop the bus traffic directly. That would reveal a lot more information and in digital form.