Supermirror neutron guides have become a standard solution to transport neutrons from the source to the instruments. The performance of the guides depends strongly on the reflectivity of the coating, the waviness of the substrate, and the alignment of the guide elements. We have performed Monte Carlo simulations in order to quantify these influences for non-perfect, real neutron guides and to compare the importance of the different loss mechanisms. The importance of a good reflectivity was demonstrated in previous publications and is supported by our investigations. We show that the waviness of the substrate can easily be compensated by increasing the critical angle of reflection without degrading the divergence of the outgoing neutron beam. For geometrical imperfections such as off-sets of the individual guide elements or deviations from an exact rectangular guide cross section, the degradation of the neutron flux at the end of a neutron guide corresponds roughly to the proportion of the cross section affected by these imperfections to the total guide cross section.