The comparison is a bit flawed since there is no dialect continuum between spoken Mandarin and Cantonese, and there might have never been one.

A crucial difference is that there is no standardized form of written Cantonese. There is a common set of characters used in advertising, sometimes newspapers, or in court transcripts, but that's it. For anything official, Modern Standard Written Chinese is used.

People write Cantonese in private of course, but that usage is not uniform at all since there is a lot of slang (especially swearing words) for which more than one popular way to write it down exists. Quite often the characters chosen for that purpose had different meanings originally. Some particularly novel or distinctive words and idioms are even partly written in Latin letters, like 快-D.