Shimano has worked on this; both the regular Nexus-8 and the Red Band have gone through three upgrade revisions since initial release.
Mind you, I don't know if or by how much the hubs' sealing has been improved, but Shimano has tried to address this.
tcs
here's a question.
How often do the bearings on a Nexus hub have to be regreased (if they have to be greased at all)?
My old hubs had a re-packing about once a year, but aren't the Nexus bearings sealed and do not require annual re-packing?