magnetism
I was thinking of magnetism doing strange things to steel components and causing for instance steel debris, from normal wear and tear, to accummulate where it should not and potentially cause more significant problems.
I can appreciate that waving a magnetic tool around in an avionics environment might induce currents to flow where they were not intended and cause damage.
I have heard also of a fuel sender where the track of the potentiometer was polluted with steel particles, attracted by induced magnetism in the structure, and that then gave a false low fuel level indication.
Of course the convenience issue of using a magnetic bit holder is a major one. But I wondered if such tools were in fact permitted or prohibited by the authorities, perhaps by means of workshop standards or similar and of which I am not aware ?