In this study, we evaluate the robustness and nonlinear (NL) responses of two variants of AlN/GaN HEMT technology under RF-step stress at 10 GHz. We develop an original NL behavioral modelling approach that includes the self-biasing effect within a unique expression. This method is applied to various gate-to-drain (LGD) HEMT geometries of the two AlN/GaN processes under study. Our investigation highlights several insights into this promising technology: including remarkable RF robustness demonstrated by handling 10 dB of gain compression without any degradation. We observe that devices with an AlGaN back-barrier exhibit higher drain saturation currents (IDSsat) as LGD increases, whereas those without AlGaN back-barrier show a decrease in IDSsat. Consequently, devices with an AlGaN backbarrier present better charge stability within the 2-Dimensional Electron Gas (2-DEG), whereas an earlier gain compression appears for those without back-barrier, and with larger LGD. Additionally, slower degradation kinetics are observed in devices with an AlGaN back-barrier before fatal failures. We identify the limitation of the Schottky gate to withstand critical RF power (18 dBm), and this gate RF limitation represents a fatal failure mechanism common to both variants.