Karlee, I understand that there's a lot of mistrust in our environment. Im not surprised that there are warnings about this. I even just looked up on the Internet. Most of the warnings are about microbes.
Microorganisms from distilled water will gain no foothold in an active living soil. Microorganisms are very environment specific. If the water picks up a plant disease, it could affect the plants, but this is extremely unlikely as the organism that would affect a plant has no reason to be in the water, or survive there.
Organisms that could be harmful to humans may exist in the water for a short time, but the issue is really with the tank and the collection surface. But even if the water manages to become contaminated, it will have no mechanism to transfer to the upper part of the plant except through splashing.
The only actual concern might be small amounts of metals leached from the coil. We're all drinking water that has traveled through miles of said pipe.
So why all the warnings? Because someone, or many someones got sick after drinking some contaminated water that came from a dehumidifier tank. Tank is probably the key word here. Depending upon the environment that the equipment is operating in, there might be a buildup of microbe food on the coil, and in the plumbing and tank.
It is always best to heed warnings. So I can't recommend that anyone use this source for drinking water. However, most modern dehumidifiers have a direct drain option, and if the flow is kept fresh and clean, the water should be fine for watering plants.