E.g. on B2 if you wanted to retrieve data to do your own scrub/validation it would cost you the equivalent of 10 months of storage just to do one retrieval: $0.005/GB/month to store, $0.05/GB to download.

Google Cloud Storage Nearline has the same problem: $0.01/GB to store, $0.12/GB for egress. But at least in this case you can egress for free to Compute Engine, so you would only need to pay $0.01/GB for retrieval.

So it's not possible (at reasonable cost) to do your own validation of what's stored in B2. In Google's case, as long as you're willing to use their cloud computers, validating your data once a month doubles your cost.

In conclusion, you're trusting the vendors to handle failures, it's very expensive to check your data yourself.