ROM Hack To what extent does Nintendo detect modified saves?

[Alexxxmoment]

I've been following the common rule of thumb "Do not hack online, it's pretty much one of the only ways to get banned besides playing a game before release and having illegal items," however I was wondering to what extent this is true? For example, if I modified a Pokemon Y or Animal Crossing: New Leaf(Welcome Amiibo) save with completely legal items, and only changed currency values(or legitimate pokemon), and then proceeded to play online, would this be detected as a hack/modified save?

A prominent member of a large homebrewing discord has said that any modifications to the save are detected, however, I doubt this. Is this true?

Thanks so much and have a wonderful day!