I would like to point out there are only two vectors to sneak equipment in:
Bring in/tamper with a set-up designed to enable cheating by having such a set-up pre-built. This is pretty hard to do in practice; you can't control what set-ups you play on in tournament (good TOs tend to assign set-ups), and tamping with set-ups requires privacy which is very hard to get in a tournament setting. With hacking, similar gimmicks were very possible in Melee and Brawl as well (give me 5 minutes alone with a Brawl set-up if I'm motivated to cheat, and that game will work in so many little ways to my advantage you'll never catch), and no one pursued them because it just wasn't worth it.
Import a built from 3ds including such an illicit build while no one is looking. Since importing is normal, this might slide, but since the import GUI shows equipment effects, no one has to be around looking when you do it, and of course any competent opponent will watch you import your stuff. I'm pretty sure negative state equipment with no effects is not possible to generate via the RNG, and even if you could, it would seem to be entirely to your disadvantage to use a negative stat build with no offsetting bonus effects which makes the incentive to cheat awfully small (cheat, risk DQ, have a harder time winning?).
So yeah, both avenues are very hard to actually pull off.
I agree that any form of deliberate effort to cheat should bring a ban for the player, whether it's using equipment, lying to naive opponents about tournament rules (note: different from being mistaken, generally very easy to tell), tampering with an opponent's controller, using an illegal controller (turbo, macros), attempting to falsely report match results, inducing a power failure on the set-up to avoid defeat, stealing from the pot, bracket manipulation in its many forms, or really any other clever scam that attempts to get ahead in a tournament by any means other than winning smash matches as outlined by the tournament rules. We can't tolerate cheats in this community, and honestly, the only reason anyone would dare try half of this stuff is out of a belief that there is anything but a super harsh punishment awaiting them if caught. That being said, I don't think cheating with equipment is particularly simplier or easier than the many other possible avenues to cheat, and it doesn't need special attention as a security vulnerability opened up by allowing custom moves. It's very, very hard to get away with on Wii U, and a reaosnable anti-cheating policy quickly makes attempting it insane.