Re: The truth is tape is more reliable
As as shop that did the move from tape to disk 6 years ago, I do confirm that the operational experience here is that disk is more reliable than tape. We no longer experience things like "not this backup, it's corrupted/incomplete/eaten by the dog, take the previous one", or wait hours to apply incremental dailies.
The BER issue (assuming there is one) it totally hidden by RAID 6 + replication.
As of cost, the whole driver for the project back then was cost reduction.