[hnfong]

Segmentation in Mandarin is easier due to tendency of the language to use 2+ characters for words. With a high quality wordlist you will go a long way.

The problem with proper nouns is that they don't end up in dictionaries, same with slang and other terms that for reasons don't end up in dictionaries.

The additional problem with Cantonese is that there's a larger class of words where the constituent characters can move around as if they were words themselves. Even for a native speaker with some experience in lexicography, it can be difficult to determine word boundaries as there are many cases where a word with characters X+Y can be interpreted as just word X and word Y with some idiomatic meaning. This issue is more pronounced in Cantonese because there are more single character words in active use.

I've actually done this before. My experience is that naive segmentation on Mandarin text with wordlist is probably 80+% accurate, while using the same algorithm in Cantonese text (with cantonese wordlist) will definitely end up "wtf".