The GameSeek Adaptor is just what I needed. Works with Nintendo Switch even after the 5.0.1 Update. Great for MarioKart multiplayer although it doesn't seem to recognise 3rd party GameCube controllers however I'm sure this the Switch's problem not the adaptor itself. It's a pretty cheap and nasty 3rd party controller so it might be that. Lacks buttons that the Switch has so be mindful of that. Not all games will work with fewer buttons.
Really good purchase highly recommend it to anyone looking to play the GameCube Dolphin emulator or smash bros on the Wii u. My GameCube controller works perfectly in there without any problems, yep buy this for the real GameCube experience