Rain barrel water usage.
There are many factors that affect the quality of collected water – roof composition, animals on the roof, water storage conditions, etc. Limited studies have been done on this. Here is some information from Rutgers University. https://njaes.rutgers.edu/fs1218/
It becomes a personal choice based on your level of comfort. Certainly, if you do decide to use it, we'd recommend you water at the base of the plants, right into the soil, as opposed to the leaves. Actually, this advice also helps to prevent plant diseases too. If you are not comfortable using it on your edibles, feel free to water your other flowers and landscape plants with rain barrel water.