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It depends on the philosophy that guides the emulation author. For example, the creator of bsnes believes in accuracy over everything. He's proud that his emulator will show the same bugs as the console when other emulators don't.

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I would also add that practicality definitely comes into play. For example, there's a GBC emulator for the GBA that is written entirely in ARM assembly because that was the only way to make it run fast enough. There is a certain amount of accuracy that was sacrificed to make it work, but the fact that it actually runs games at playable speed is notable in-and-of-itself. It probably could have been done more accurately, but that doesn't really matter to most people if the games run at completely unplayable speeds, accurate or not.

One thing I always wonder about low-resource-usage emulation projects for old consoles with fixed game libraries (e.g. GBA emulating GBC), is why you can't just throw some heavy ahead-of-time analysis at the problem of statically recompiling the whole game library for the native destination architecture. Why write an "emulator" at all, when you know exactly what code you want to run, and know exactly what code that code should translate into when run on the target CPU?

I guess a potential problem would be that this requires shipping a (IP-infringing) copy of the recompiled game library with the emulator. But it doesn't quite: instead of a library of ROMs, you could just ship a library of hint files to turn the static-analysis step of the recompilation into a few seconds of work, such that you still need an input ROM and the dest-ISA program is only generated in memory.

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