It is known that slow light propagation in disordered photonic crystal channel waveguides leads to backscattering and localization phenomena. The knowledge of the reflection of a slow light mode at a single disorder defect of the periodical structure can help to estimate the backscattering intensity and the localization length. Here, this Bloch-mode reflection is calculated in a simplified slow light waveguide using an eigenmode-expansion approach. We show that by properly engineering the waveguide, backscattering can be significantly reduced while maintaining the same low group velocity. A strong effect of the mode's anticrossing taking place in photonic crystal line-defects is demonstrated on backscattering. The localization length of slow light waveguides is estimated, which provides fundamental limits for the applicability of slow light waveguides.