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I did so much research, looking over Consumer Reports and reviews when I was looking to replace my humidifier. None of them really had the options I was looking for, so it felt exhausting. I finally bit the bullet when a Buzzfeed article had a list featuring a humidifier. I bought it, and have been very happy with it! Easy to clean, depending on the setting (low or high), will last either roughly 24 hours or 12 hours.
I don't know why one is recommended over the other for sure, but I could imagine it's because most ultrasonic humidifiers I've seen are pretty small so they probably can't produce enough humidity for large rooms or whole houses.
I have this one for my bedroom and it works pretty well. One thing I like about ultrasonic humidifiers is that they seem to create less residue from hard water since it's basically just blasting the water into the air instead of boiling it like a steam humidifier (I have used those too, and I have to clean it out with vinegar about once a month because of mineral buildup).
I also own this model of evaporation humidifier and this does a decent job of humidifying most of the house. This is better for large areas because it's just much larger, and moves a lot of air through a large wick with tons of surface area. Before I installed a whole-house humidifier on my furnace, I used to refill this ~20 liter unit about once every day or two.
Basically I think it comes down to the size of the unit, and ultrasonic humidifiers don't get very big. I don't know if it's a technology limitation, or just because an ultrasonic emitter big enough for a 20 liter humidifier would drive you crazy with the high-pitched noise. Ever used an ultra sonic parts cleaner? It's grating, and I usually set it to run in the garage and leave to go do something else.
That's all my guesses, I'm not an expert, just basing this off observation of what humidifiers are available to buy.