Low deformational resistance of polymers can be effectively improved by reinforcement, mostly using rigid particles and fibers. In contrast, this work reveals lower tensile creep resistance of microfibrillar composites (MFC) based on high-density polyethylene (HDPE) reinforced with in-situ formed polyamide 66 (PA66) fibrils against original undrawn system containing spherical inclusions. This performance is quite different from that of other known composites containing e.g. glass fibers or spheres. Moreover, the incorporation of graphite nanoplatelets (GNP) partly eliminates this shortcoming in MFC but worsens performance of undrawn system reinforced with PA66 inclusions. The obtained results highlight so far unknown negative effect of relatively low-modulus viscoelastic fibers and GNP-modified parameters of interface on creep resistance of polymer-polymer composites.