The risk of false and malicious blacklisting of non-spammers. This is a serious problem. This is unnecessarily complex. Efforts are underway to adapt this to e-mail. The client verifies the signatures, and if it's signed by someone the client agrees to listen to, the message won't be shown to the user at all.
Clients are also available to work with news servers, to NoCeM messages on a site-wide basis.
Either you trust someone's judgement and honor their NoCeMs, or you don't, and they're completely ignored. Unless the user lists them as trusted cancellers, their notices will be ignored. There is no risk of some central database being compromised by spammers or censors. While I'm not a lawyer, I can't see any way to sue someone for merely suggesting that a spammer's mail isn't worth reading. This is not a risk.