One of the reasons why 400Hz was originally used in aircraft is that you could use smaller and lighter inductive components (mostly transformers) which had the same performance as much larger/heavier ones designed for 50/60Hz.
The main problem with using these components outside their specifications is when the input frequency is lower, which reduces the impedance so allowing higher currents to flow which can overheat the device, e.g. using 400Hz equipment with 50/60Hz input = bad.
Modern SM power supplies are tolerant of input voltage/frequency within quite a wide band because of the way they work.
I would speculate that non-approved devices are disallowed from using power outlets on the flight deck not because they are expected to burst into flames but because of the possibility of a fault causing an fire in the supply wiring, which would be inaccessible in flight. Power sockets designed for in-flight use have much more in the way of protection built in, compared to the ground service ones. It may be as simple as that.