It was only last year that Nintendo fired off a barrage of lawsuits against Nintendo Switch emulators such as Yuzu, as the company claimed the emulator was "facilitating piracy at a colossal scale."

Despite Nintendo's actions against popular emulators, the company's patent attorney and deputy general manager of Nintendo's intellectual property department, Koji Nishiura, said at the Tokyo eSports Festa earlier this week that emulators are technically legal. Nishiura gave the statements alongside property leaders from Capcom, Koei, Sega, Konami and others.
While Nintendo's top IP lawyer agrees emulation is technically legal, there are ways for emulators to cross over into illegal territories, such as the circumvention of the Switch's "technical restriction measures," which prevents the console from playing pirated games. Another example of an emulator going too far is when it includes specific programs that are subject to copyright protection. An example of this would be Nintendo's home screen or menus.
According to Nintendo's own IP lawyer, it seems emulation itself isn't illegal, but how it's implemented, and it appears that even the slightest step in the wrong direction will result in a lawsuit arriving in the mail of the owner of the emulator. As for the emulators Nintendo has shut down, Nishiura said these emulators bypassed technological restriction measures and that any emulators directing users to services or marketplaces that house other copyrighted material are also against the law.