NSBM's a difficult problem. For one thing, where do you draw the line? For every band that are out-and-proud unequivocal neo-nazis preaching race-war, there's another who insist they're apolitical as musicians, another who just want to "explore themes of our national history" (yeah, pull the other one...) and probably half a dozen who just flirt with the imagery because it sells records.
For another thing, this stuff is interesting - there's no two ways around that. Musically, it's almost entirely straight-up bad (when Varg Vikernes is the best a movement has to offer, you know there's a quality problem), but the cultural mechanisms that made it and the social history that feeds it is, speaking with cold clinicism, really very interesting.