There have been incidences of failure of microwave ovens mounted above ranges due to entry of steam. Usually the insulation in the transformer fails, or excessive leakage builds up on the timer PCB. It would all depend on where the vents are on the particular microwave oven, but in any case the steam generated within the oven is allowed for and usually exits via the most direct route from the top of the cooking cavity.
I just have mis-givings about having a metal framed appliance that can generate in excess of 3kV being wet from steam, after all, whose to say that the grounding on the microwave hasn't broken down after years of use.
And the fact that some people have a habit of cutting the ground pins off of plugs to make them fit (I realise this isn't the norm in most cases).