Ah, reminds me of the time I tried to brew a Christmas Ale with snow. Bad idea. All the pollutants in the atmosphere made the final product taste bad, real bad.
That is part of the beauty of the rainwater; raw from the tank it is practically tasteless, boiled it is absolutely tasteless. But, once again, galvalume roof, gutters, first flush and settlement. Think what you will, but herebouts RO tapwater is horribly salty and ruined the beer. Rainwater makes good beer.
I understand why this idea comes up. Many Caribbean islands use rain water from their roofs and store the water under their houses.they then pump some of the water to tanks on the roof to be solar heated. apparently their air quality is much higher than ours. Poorer countries don’t have the utilities of more prosperous countries and have found other ways to cope.