The controllers, mostly, for me. Which Just Work and don't decide to become unmapped for no damn reason (admittedly less common on Retroarch/Lakka systems than others, though). I mean those things are
legit. About as close to the originals as you could hope, and I've side-by-sided them. Only real loss on the NES are the four-score games (Super Spike V'Ball and World Cup Soccer are both
actually good and are
actually good with four players!) and light gun games, of which I love two but would only really defend one as even sort-of worth a damn (Duck Hunt). Still don't get why that wasn't on the Wii. SNES classic, on the other hand, is damn near perfect. After you hakchi all your other games on, in both cases, obviously.
NES controllers especially are great for kids, for whom even SNES controllers are confusing with their four face buttons plus two shoulders—and I remember, as an NES kid who didn't get another console until I bought myself a PS2 in college, finding anything with more buttons than a 3-button Sega Genesis controller kinda tricky at first, though I eventually managed well enough to hold my own on some N64 games at friends' houses. Only "complicated" controller I've found to be almost as usable for my young kids is the Gamecube controller, so they got that right after all I guess.
Sure you can get knock-offs, but Nintendo doesn't screw around with controller quality, even on cheapish emulator systems.
[EDIT] also I've owned two Wii-connector-to-USB adapters, both from Mayflash, and both were janky and soon stopped working at all, so just buying the controllers (or just using the Wii Classic Pro controller, which is what I actually attempted those times) hasn't given me acceptable reliability.