DB for cities, countries and addresses
Google's address data is probably one of the best maintained data sets. Still unlikely to be flawless.
1. Services close/stop (Picnik as the image editor in old c5 versions)
2. Services change (API versions become deprecated)
3. Services rate-limit (Anything google related for example requires a per site API key to use)
For this type of data, to get something remotely accurate you probably have to buy it off of some company.