You have already highlighted the benefits of the self-moderated thread in a good precise way however every good thing can be misused and some scammers and spammers use self-moderated threads for this purpose.

1- A Scammer can open up a self mod thread about his service and would be able to delete all the negative feedbacks which people may wrote about his service.

2- A person may be willing to enforce his own point of view in one of his self moderated threads and delete the replies which differ from his opinion and thoughts.

Besides that i don't think there are any more disadvantages to Self Moderation threads.