Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

That the emulator behaves exactly as genuine hardware does.

For one, there are sometimes hardware quirks in consoles that are used to implement features. Like the video screens along the track in Mario Kart 64, the N64 has shared memory between the CPU and GPU, which they used to generate that effect. It's difficult to replicate that behavior in an emulator.

Then there's lag. You might argue that an emulator can provide a better experience by obliterating lag via brute force, but that wouldn't be the authentic experience the real hardware would provide.

Or a simple one I've noticed with SNES emulators: None of them get the sound quite right. It's hard to explain, there's a high pitched "rattle" that isn't present on a genuine SNES, it's almost like any emulation is too perfect and isn't sanding down a rough edge the original hardware did?