Well I guess some of you guys enjoy living on the edge a little
. Even with an old humidifier without an overflow drain, if failure occurs it still spills down close to a floor drain so no damage is done. Also keep in mind that a fish tanks isn't exactly the same as a humidifier. In the tank environment it's much more vulnerable since it isn't as clean and things can jam it up or stop it from closing.
Simplicity is better as a rule of thumb within reason (a car is still better than a horse) but mechanical floats do fail so I think it'd be pretty foolish to hook up one to an actual aquarium from an unlimited water source, if you've got a separate container for top off which wouldn't harm the tank if it was all introduced that's fine but a direct RO hook up is an accident waiting to happen. A solenoid and a timer can be easily setup for less than $100 which is very cheap insurance considering what has already been invested.