How SNES emulators got a few pixels from complete perfection

It is interesting to me how some systems are easier to emulate than others, and the ease of emulation seems to correlate somewhat with ease of programming model. The infamous example which comes to mind even far beyond the SNES is of course the Sega Saturn, while on the other end of the spectrum, Dolphin has support for multiple consoles at once.

Nintendo Switch Online subscription includes an "official" SNES emulator. How does it compare to BSNES, and would a decompilation of the emulator help with resolving the PPU issues?

Dolphin has support for two consoles: Gamecube and Wii. It supports both of them because the architectures are extremely similar. I don't get what you mean by "ease of programming model". Saturn/Playstation/N64 were the first to have games programmed primarily in C with libraries provided by the system developers. This generation is harder to emulate simply because the system architectures contain more chips connected with wider buses running at much higher clock speeds. All of those factors make low level analysis, like with a logic analyzer, impractical. Another factor is the popularity of the system and level of interest in emulation of the system. The original XBox is a good example of this. Gamecube and PS2 have much more polished emulators because they have more titles that interest people, while little effort is spent on emulating the original Xbox (despite being essentially a PC) arguably due to the lack of exclusive titles to make it worth it.

Nintendo's emulators are usually equivalent to what the state of the art was 15-20 years ago. They have plenty of bugs, usually fixed with duct tape and game specific hacks. They are designed to run the set of games they sell and nothing else -- once you get out of what the QA team has explicitly tested, it's not uncommon to find unemulated or badly emulated hardware features.