Until your thread i never knew this was even a thing.
Can we get a photo?
A little reading around the subject i cant find anything that suggests the equipment is harmful in itself. It does create more evaporation which might lead to limescale build up on the aquarium above the waterline, and if you are topping off evaporation you should use RO
or distilled water to prevent disolved solids building up in the water.
Edit: There is some cause for concern that ultrasonic humidifiers in general might have some health issues for people as the humidifier effect will aerosolise bacteria as well as the water, which you would then breathe in. The article i read accepted issues would be rare if the equipment was regularly cleaned. This would be more difficult in an aquarium situation where bacteria will be rife in the aquarium water.