Controlled surface-initiated atom transfer radical polymerization (SI-ATRP) has previously been described as a versatile method that allows grafting polymer brushes on purely cellulosic forms of nanocelluloses, i.e., cellulose nanocrystal (CNC) nanorods and bacterial cellulose (BC) networks. However, corresponding SI-ATRP on long and entangled cellulose nanofibers (CNFs), having typically more complex composition and partly disordered structure, has been only little reported due to practical and synthetic challenges, in spite of technical need. In this work, the feasibility of SI-ATRP on CNFs is exemplified on the polymerization of poly(n-butyl acrylate) and poly(2-(dimethyl amino)ethyl methacrylate) brushes, both of which showed first order polymerization kinetics up to a chain length of ca. 800 repeat units. By constructing high and low initiator densities on CNF surfaces, we also show that, surprisingly, a higher grafting density of polymer brushes around CNF causes noticeable degradation of the CNF nanofibrillar backbone, whereas lower grafting densities retained the structural integrity of the CNF. We tentatively suggest that the side-chain brushes strain the disordered domains of CNF, causing degradation, which can be suppressed using a lower degree of substitution. Therefore, SI-ATRP of CNFs becomes subtler than that of, for example, CNCs, and careful balance has to be achieved between high density of brushes and excessive CNF degradation.