Comprehension of untoned speech in Cantonese Do Cantonese speakers still understand speech when all syllables are spoken in a neutral tone (similar to the Mandarin's 5th tone)? And to what extent do they understand it?

I spoke to a Cantonese waiter and I was particularly tired at that time so I was unable to articulate my tones and they seem to not understand anything at all.

I heard somewhere that if you forget the tone it's safer to use a neutral tone to avoid saying bad words and the like.
Depends. if you have all tones incorrect, then it's hard to guess what you are saying, but if you have only 1 or 2 incorrect, generally people can understand you. For example, if I say, I duck your mom. You probably get the meaning. but if I say, I duck your nun. you will be confused.