How are the emulators on this thing? It plays DS games natively, but I want something as a single solution for NES, SNES, and GBA as well (obviously for things not available on the eshop).
It also runs GBA games natively (roms, not carts, obviously). It actually switches to GBA mode (where you lose 3DS functionality, like accessing home screen and sleep mode, just like playing GBA carts on DS or DSLite). So in terms of accuracy it's better than any emulator, except I think Nintendo added a dark filter to some games, but you can download versions where people have fixed that.
SNES emulation is fantastic, but I encountered one issue with games not on the eShop (rom injections into Nintendo's official emulator), there is no save state/restore point functionality.
NES emulation is OK. I don't like the dark filter applied in the official emulator, or lack of pixel-perfect mode.
Alternatively you can use other emulators like RetroArch cores which offer a lot more features and options and allow you to emulate more platforms, including NeoGeo, CPS1, CPS2 and even PSOne (although performance isn't great on this one, maybe it will improve).
I have compared the same GBA and DS games running side-by-side on a DSLite and N3DS XL, and even when running them at 1:1 resolution on the 3DS to disable upscaling, they still looked softer and the colors were not as vibrant as on the DSLite. My N3DS has TN screens (and I suspect the DSL had IPS), so that might be the reason for duller colors. But besides muted colors I personally always hated how 2D looks on the 3DS top screen, most people don't seem to have an issue with it though since I keep hearing people always turn off 3D even for 3DS games, which I cannot fathom (the difference between 2D and 3D on the 3DS is like going from SD to HD to my eyes). This might be less of an issue on the non-XL units.
Mega Drive/Genesis emulation is fantastic on it too, thanks to M2's efforts on the Sega 3D Classics. Playing Sonic 3 on it is glorious (which isn't officially released).