Electrochemical and photoelectrochemical reduction of nitrogenous small molecules, including N₂ and NO₃^-, represent appealing routes for the production of NH₃ under ambient conditions. However, major challenges include the activation of highly stable reactants, avoidance of spurious artifacts introduced by contamination, control of product selectivity, and suppression of the competing H₂ evolution reaction (HER) in aqueous environments. In this work, we comparatively investigate the activities of Cu-based oxide semiconductors, CuO, Cu₂O, and CuBi₂O₄, for the photo-assisted N₂ and NO₃ reduction reactions (N₂RR and NO₃RR, respectively), taking advantage of their p-type characters, moderate bandgaps, and suitable band edge positions. The ternary compound CuBi₂O₄ is specifically selected for its improved photoelectrochemical stability compared to the binaries. The Cu-oxide electrodes were synthesized via electrodeposition methods and the photoelectrochemical investigations were carried out in alkaline aqueous electrolytes. Notably, all photoelectrochemical investigations were performed in an optimized workstation, where background contamination (NH₃, N₂H₄, and NO_x) are carefully evaluated and high purity is ensured. Although all three Cu-based semiconductors were found to be photoactive, we observed no production of NH₃ when tested for N₂RR. Moreover, during photoelectrochemical NO₃RR, Cu-oxide photocathodes showed major selectivity for NO₂^- rather than NH₃. Although the selectivity for NH₃ increased for Cu₂O at decreased positive potentials, NO₂^- remained the major product of this reaction. Furthermore, photocorrosion leads to a rapid loss of photoactivity for the Cu-oxide photoelectrodes. While CuBi₂O₄ showed improved stability, photoelectrochemical activity was found to be reduced. However, complementary electrochemical measurements of Cu single-atom catalysts hosted on carbon (Cu@C) show appreciable activity and stability for NO₃RR, yielding the two major products NH₃ and NO₂^-, fully suppressed HER, and an NH₃:NO₂^- ratio of 3:1 at an applied potential of -0.53 V vs. RHE. Overall, these results indicate that Cu-based oxides are poorly suited for NH₃ synthesis via N₂RR and NO₃RR under aqueous photoelectrochemical conditions but that Cu@C may provide promise for dark electrochemical NO₃RR under otherwise similar conditions.