Also, on Sakurai thinking this feature is broken, it's probably because it allows players to give tools to certain characters that they weren't balanced around in the first place.
Like, let's assume that Mario's Electric Cape stuns enemies, similar to ZSS's down smash.
Mario isn't exactly a character who's lacking in KO power. His smash attacks are more direct and less awkward than ZSS's, and if he lands that against a grounded opponent, he's most likely going to get a half-charged smash attack off it, which is really nasty when you consider how wide the hitbox is on his cape compared to ZSS's down smash.
That's just one example, but ultimately it's going to come down to just what everyone can do with their modded specials and how strong the options are. Whatever the case, I see no reason why we can't have two separate metagames - one for vanilla smash and one with customized moves.
Perhaps the stars will align and everyone will be so broken that it's balanced, just like Mahvel. Who knows?