The fatigue behavior of the lead-free piezoelectric Bi1/2Na1/2 TiO3-BaTiO3-K0.5Na0.5NbO3 (BNT-BT-KNN) under bipolar and unipolar electrical loading is investigated. Special attention is paid to the effect of a decreasing ferroelectric order which was affected by an increase in KNN-content. It is found that, in mainly ferroelectric systems with low KNN-content, bipolar cycling leads to domain wall pinning and formation of micro-cracks, while in mainly non-ferroelectric systems with high KNN-content fatigue can be rationalized by the repeated field-induced phase transition occurring during each field cycle. Under unipolar load, fatigue in low-KNN systems is dominated by domain wall pinning and the formation of a space charge field. Only the latter effect is present in non-ferroelectric high-KNN systems, which fatigue to a smaller degree.