If you're competing in Smash Ultimate tournaments, there are a number of specific rules you need to follow to be eligible to play. One of these rules is directly related to controllers and to be more specific, what controller models are legal.
There are in fact a number of controllers that are legal for Smash Ultimate which include; GameCube Controller, Switch Pro controller (with battery removed), Wii Remote, Wii Remote and Nunchuck, Wii Remote and Classic Controller, and the Nintendo 3DS Family (in controller mode).