People learn faster than machine learns.

If people want to post hateful comments, they will find ways to circumvent any AI model that is thrown their way, they will create lingo that confuses the model but that everyone will identify quickly as hate speech.

Also anyone can appropriate any symbolism to their own speech and AI will take a lot of attention and training to catch up, see Pepe the Frog (https://en.wikipedia.org/wiki/Pepe_the_Frog) as an example.