I'm thinking that part of the problem which (extreme) metallers may have with Neurosis is that metallers are used to guitar playing which falls between two conventional poles of style; the technically proficient (best exemplified by technical DM), and the simplistic melodic (best exemplified by certain norwegian BM bands). To my mind, Neurosis display neither technical ability nor even a command of simple melody. In my opinion they seem more concerned with the guitar as a rhythmic and tonal intrument. They use very rhythmic riffing, accentuated by having very little in the way of melody, to command strong senses of drive, force, peace and calm. As for the tonal side, they generally use assonance and dissonance of tone, rather than melody, to give atmosphere. The tribal, almost poly-rhythmic style of drumming, accentuates former side of the guitar playing whilst the use of droning instruments, such as organ-synthesizer, violin and bagpipes, accentuates the latter.
I think this approach to guitar playing has a lot more to do with their hardcore roots than their metal ones. Personally I think their guitar playing is strongly influenced by the emphasis on driving rhythms found in the likes of Discharge and Doom, and also the use of almost melody-free riffing for song bulding by Amebix or Axegrinder. In fact, I hear a lot of Amebix in Neurosis.