No matter how properly installed and maintained they may be, that doesn't alter the fact that having lots of earthed metal in close proximity to potentially live parts creates a hazard - particularly (given that this is a DIY forum) in relation to non-electricians who might sometimes be 'fiddling in CUs' without adequate knowledge and/or care. I would personally prefer not to possibly be reliant on paint to save my life!Properly installed and maintained metal ones are fine.
As I often write, if it hasn't happened already I feel sure that it's only a matter of time before someone dies because a CU was ('unnecessarily') metal ... and, as I recently reminded people, the regulation requiring CUs to be 'non-combustible' (impossible though that is!) is plain daft, anyway, because it does not require any 'fire containment'.
