2 Answers

General dehumidifier water is considered a rather clean kind of greywater: not suitable for drinking, but acceptable for watering plants, though not garden vegetables. The concerns are:
the water may contain trace metal from the solder, most significantly lead (which is quite damaging), but also copper, aluminium, and zinc;
various pathogens accumulate in the water, particularly due to its stagnancy, including fungal spores; unlike in distilled water, the water is not boiled, which would kill pathogens (including bacteria);
as with distilled water, minerals are largely absent, hence it is somewhat flat tasting.
The trace metal poses a danger if used on edible plants, as they can accumulate; however, the water is otherwise usable for irrigation.
One can make food-grade dehumidifiers (avoiding toxic metal and keeping the collection tank clean), which are called atmospheric water generators.