Processes disclosed are capable of converting biomass into high-crystallinity nanocellulose with low mechanical energy input. In some variations, the process includes fractionating biomass with lignosulfonic acids, to generate cellulose-rich solids; and mechanically treating the cellulose-rich solids to form nanofibrils and/or nanocrystals. The strong lignosulfonic acids created during delignification give a pH less than 1 and hydrolyze preferentially the amorphous regions of cellulose. The total mechanical energy may be less than 500 kilowatt-hours per ton. The crystallinity of the nanocellulose material may be 80% or higher, translating into good reinforcing properties for composites. The nanocellulose material may include nanofibrillated cellulose, nanocrystalline cellulose, or both. In some embodiments, the nanocellulose material is hydrophobic via deposition of lignin onto the cellulose surface. Optionally, sugars derived from amorphous cellulose and hemicellulose may be separately fermented to co-products.