As an Ex-plumber who has had occasion to swim around in peoples rain water tanks, I would be wary of the water.
Just a couple of things that come to mind, other than the self evident one about bird poop, and tanks being some form of heaven for green tree frogs:-
If the water was pure the pH would be 7, by definition.
There are still plenty of leaded cars out there; Tetraethyl lead is bad shit ok.
The most Carcinogenic compound known can be found in diesel exhaust.
Urban particulate fallout includes all the crap we love breathing including, but not limited to the above, powdered concrete, paint particles, metals, oxides of nitrogen, some very strange sulphur compounds and more.
Ok we leave the bacteria count alone as yes we are going to boil the wort - there are still lots of reasons to be careful, at a minimum I would carbon filter the water, ideally particulate filter, carbon filter then boil.
Honestly in most cases I dont think you will have any problem, just think about where you live, and what's in the air, there are places where you can get reports on atmospheric pollution (this is what will be in your water tank). Baring that find a tank thats been in use for a couple of years, make up a long dipper and see how much muck you can dredge up from the bottom.
It would be brash to blithely assume the water is OK.
MHB