I'm thoroughly unconvinced.

Doing some basic research... hard water is overwhelmingly various carbonate and bicarbonates of magnesium, calcium, sulfur, iron, maganese, and aluminum. All of which are essential nutrients and readily soluable in water.

The other proposed problem was pathogen aerosols- however I was unable to access anything but an abstract. So, I don't know if they survived being aerosolized, produced more and/or worse pathogen count than evaporative humidifiers, Nor the size of the pathogens.

It seems to me the known risk is mostly mechanical (Asthma, exacerbated COPD, etc) and nonpersistent (particles dissolve and are used or excreted via the same pathways as when consumed). With an unknown risk on the pathogen side.