it's a reputational and data quality issue as well. completely decentralized content distribution doesn't usually work unless there is some kind of curation, there are just too many motivations for disruption. it's not an impossible problem, but long ago ceased being mostly about the technical occupation of shipping bits between nodes.
You could always have sites that only serve up indexes of NZB files (e.g. "NZB file => SHA-256"). Then you could use these to determine which NZB files to trust. I would assume that distributing a "NZB => Checksum" hash would be a lot smaller/easier than running an entire site on top of a terabyte of NZB files.
