I have a Smash branded GC controller. It came in an official Nintendo Switch branded box, has the Smash logo in the middle and it has the standard GC connector as well as an extra long cable. Otherwise, it's functionally identical to the original controllers I have. You do need a 4 player adaptor with 2 usb connectors to use regular GC controllers on the switch.
The annoying part is that the weird GC A button position (central, instead of on the right edge) means that a person who plays Smash with the standard Switch controllers won't have an easy time using the GC controllers and vice versa. In my experience, most younger players I've encountered seem to dislike the GC controllers for that reason. I wouldn't invest in GC controllers unless you're certain that they'll be used. Non-pro players probably aren't going to be able to do all that wave dashing and other advanced techniques anyway.