The occasional piece of spam can be deliberately allowed through for use as a target. Requires less effort than manual moderation. Eli the Bearded, the moderator of alt.sex.stories, is willing to share his moderation bot expertise. Requires considerable work by one or more people on an on-going basis. Difficulty in getting news servers to accept moderation of already existing group.
Spammers can and do forge approvals.
Slim but existant possibility of censorship. Servers which do not accept the moderation will continue to propagate spam. Propagation of messages outward tied to site where moderation is done. Possibility that mistake made by moderation bot will be missed and on-topic accidentally discarded, or spam allowed through. Would prefer creation of new group to moderation of existing one, but will probably grudgingly concede.