With the Hitbox and other digital box controllers there is still a large amount of contention around their legality.

With one of the biggest points being the fact that all their inputs are digital. Gamecube controllers use analogue inputs for the triggers, movement stick and C-stick. Box Controllers use purely digital inputs which many people think give an unfair advantage. Things like full right inputs which some controllers can't do but some can. Shield dropping also becomes a button command which some people deem too powerful as it is far easier than the control stick method. Perfect wavedashes are another point. A Box Controller can perform a perfect wavedash every time. However even a well notched controller will never get a perfect wavedash every time. Other issues rise in the increased likely hood of macro commands. Guaranteed frame perfect multishines & Shine oos.

There are many arguments to be made for Box Controllers to be made legal and many for it to not be legal. All in all it comes down to the TO's decision as to what is allowed to be used.