Depends on who you ask. What’s the region in question? What counts as a standard? Is Nynorsk codified by ISO? Et cetera.
> People write Cantonese in private of course
It‘s used plenty in public in Cantonese-native areas!
> that usage is not uniform
Yes, there is a disconnect and different ways of writing down a turn of spoken phrase. Sometimes people wouldn’t know offhand how they would even do it about a purely spoken turn of phrase.