The biggest problem are bacteria in my opinion. Even if you clean it all the time (which you won't because it's pain in the ass) and you have high end one with filters and anti bacteria mechanisms - it can still be pretty bad (and becomes expensive so you can buy different type).

Its like if you were drinking that stale water thats been there mingling with all the bacteria all day.

My close friend (oterwise perfectly healthy and fit) got really nasty breathing issues along with infection and fever. He couldn't get rid of it for weeks thinking it's COVID related. Turns out it was his high end humidifier he has for plants. Once he turned it off it was gone in a day never to come back. He swapped for some evaporator one and never had single issue.

Turns out you can find many articles about this if you search for it. Some countries are even thinking about banning them.