The basic "problem" with this approach is that the customer (be it a client or an agency) can't be 100% sure if the view data you deliver them is correct, they

The basic "problem" with this approach is that the customer (be it a client or an agency) can't be 100% sure if the view data you deliver them is correct, they can only count the clicks. If the ads are on your server and there is no external content involved, only you get the stats. It's not a problem only for in-house ads, or when the customer doesn't care about the views and is only interested in clicks.