Lots of folks use in ground cisterns to capture rain water. The only key is to ensure that you are screening the water before it enters the tank. This can be done at the downspouts or even at the top of the tank. Screens need to be removable so they can be cleaned. Works like a charm
This works fine for irrigation water, but the screens do not make the rain water safe to drink.
Think about how long your roof goes between rainstorms. Dust, dead bugs and bird doo happen. In some places small rodents, mice, rats & squirrels also scamper across the roof. All of this gets washed down with the rain water. Screens will keep larger debris out of the water, and prevent clogging the distribution system, but do not remove smaller debris and bacteria.