I agree with your logic Chris, especially regarding the advantages of the softness. Dan is also correct regarding bird poop and such. Should be addressable with a sufficient filter system. I am going to employ a similar rain water system on my land, thinking a sand bed followed by a charcoal bed should (might) provide adequate filtration?..
Isn’t it amazing that rain can just filter through the ground in most places, and be perfectly fit to drink?
As a final safeguard, I intend to use a ceramic filter cartridge, guaranteed no microbes pass.
The roof itself is a concern for me. Asphalt shingles are a bit suspect, though the literature mostly rates them as safe. PAH’s seem to be the concern, along with formaldehyde leaching. I hope you have a corrugated steel roof with a bake on polyester resin painted surface. I am biased anyways, having worked with sheet metal for a career, but it truly is by far the superior roofing material. Can’t catch fire, and should last 50 years plus with no maintenance given the superiority of the new paints, and chemically inert.