The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.