We report on AlN/GaN MISHEMT technology on SiC substrate for X-band applications. Transistors with 100 nm gate lengths deliver a high off-state breakdown voltage above 180V and 110 V in semi-on state. RF small signal measurements up to a drain bias (VDS) as high as 70V for two different gate lengths (LG). The maximum oscillation frequency (Fmax) strongly increases versus VDS. As a result, Ft/Fmax of 29/276 GHz with a power gain (Umax) of 28 dB at 10 GHz and VDS = 70V has been extracted. To the best of our knowledge, this is the highest small signal power gain achieved at a drain bias of 70 V. These results are attributed to the favorable AlN/GaN epi-design for high frequency operation while inserting a gate dielectric to prevent the gate leakage current and related device degradation under high electric field. Continuous wave (CW) power performances have been assessed at 10 GHz and VDS = 50V in deep class AB operation. A high output power density of 29.1 dBm (8 W/mm) was measured at the peak power added efficiency (PAE) close to 50%. It can be noticed that no-degradation of the transistors occurred subsequently to many load pull sweeps up to VDS = 50V, reflecting the promising device robustness under harsh conditions. These results pave the way for superior X-band performances operating beyond 50 V in a reliable way.