Unfortunately even if you only ever use distilled water, crud and dust build up in the reservoir over time, and also start to grow bacteria. Technically the user manuals on these units will say something like to clean them every "use" (ie every day if used continuously), but realistically nobody does that.
Ironically if you use distilled water (even if you distill at home) it cancels out the up-front cost saving, which is about the only advantage of ultrasonic type!
Safer IMO to use an evaporative pad humidifier which physically can't atomize trace dirt into tiny pollution particles, then use humidifier drops to protect against bacteria.
It's so bad that it's cliche in AQI circles to ask "where's the humidifier" when someone complains of mysterious new dust buildup or PM2.5 spikes...
Tech Connections video: https://www.youtube.com/watch?v=oHeehYYgl28
Further reading: https://dynomight.net/humidifiers/
This storefront seems to have some good options (no affiliation): https://www.amazon.com/stores/page/43E58B1A-6C68-4879-9C38-7...