400 Hz has a definite advantage when two conditions are present:
1. There is a high penalty for weight, so the smaller motor and transformer magnertic circuits that 400 Hz allows, based on saturation, is a critcal factor.
2. The wiring is generally small enough that the higher skin effect penalty on conductor ampacity is not a problem.
Where are both conditions present? Aviation for one. Back packed or portable equipment before DC transformation became so easy for another.
I did some experiments with synchro resolvers where one identical unit (the receiver) matches the shaft angle of another identical unit (the transmitter.) They were very small and lightweight because they were designed for 400 Hz excitation. Running them off 60 Hz required a much reduced voltage and greatly reduced the output torque.