I tried using an ultrasonic humidifier in an area with 150 -200 ppm TDS water. The humidifier reservoir was clean but the area around the humidifier, in the grow area, was covered with a whitish buildup. I assume that was the minerals in the water left behind after the fog settled and evaporated. If I tried it again, I would use distilled or RO water in the humidifier.
Ultrasonics dont actually make 'steam' as they dont evaporate the water hence the term 'cold steam'. They excite the water and droplets get airborne, unfortunately carrying any minerals in it with them. So if your water has a high TDS dont use them. Cheers Steve