For DS/2DS/3DS specifically, there is no emulation hardware that has resistive touchscreen, which I think is superior to capacitive for many games. For 3DS, the same thing applies to the 3D capabilities. Additionally, I’m under the impression that 3DS emulation isn’t fully reliable for the full library, but I may be out of date.

TLDR; emulation may not be as feature complete or as quality as original hardware.