In the DR services my company provides, failover testing is baked in to the cost, and is _mandatory_ annually. It is hard and complicated and expensive, but not as hard as explaining to the customer that "we have your backups, we just can't use them because we never tested whether the software would run on the DR hardware" or "don't worry, the replica of the file server is secure in the datacenter, you just can't log in now because the Active Directory server is tombstoned and won't process your logon." When someone needs their data, what's the difference? Can I see my files or not?
It's more than a cliché that without restore tests, you don't really have a backup-- if the customer won't commit to testing the DR, we won't provide the service anymore. Anyone who pretends anything else is acceptable is kidding themselves.