I know that in theory it should be pure water, however I think you’ll find that in reality as the water condenses it takes all the pollutants with it, and there is of course no chlorine in it to kill any bugs.
I once gathered a load of pure white virgin snow and melted it with the intention of using it in a soft water aquarium. I was horrified at the dirty dark coloured water that had resulted from the pure white unadulterated snow.
The condenser itself in the dehumidifier which the water runs off, will be covered with moulds and bacteria – think of how badly the air-con in a car can smell if it hasn’t been used for a while.
The only use water from a dehumidifier has is in a steam iron.