A recent Time magazine article articulated the major downsides of humidifiers, particularly how using regular tap water (not distilled water, which is fairly expensive and hard to get), is basically spreading pathogens and bacteria that love moist places, and slowly growing in your diffuser.
"Your skin cracks and bleeds, your lips shrivel to prunes and your nose feels stuffy and inflamed. Not only is dry winter air uncomfortable, but experts say those skin changes weaken your body’s defenses against sickness-causing germs. It’s no wonder Americans buy roughly 10 million humidifiers each year.
But depending on how frequently you clean your machine—and the type of water you fill it with—you may be inhaling harmful microorganisms."
- Time Magazine, May 2017