Why plain-old Finder copy for initial backups? Because Carbon Copy Cloner was running at 1/2 or 1/3 speed for a clone. Later, incremental clone backups will of course use CCC, which is highly efficient at copying just what has changed.
Update: Finder copies have been unreliable in the past, and this time proved to be no exception; the Finder ran into some of its own bugs (seen 'em before) that killed all the copy operations and silently too—the progress dialog just disappeared with incomplete copies, no warning or errors given. More Apple Core Rot. So I’m now using CCC after all.
But a plain Finder copy is not enough to let me sleep at night after such a change: IntegrityChecker can prove-out the integrity of the data, the file count, etc.
While the primary storage uses SoftRAID, the backups were implemented without RAID and use standard Apple Disk Utility format—a decision I almost abandoned given that it took half an hour to partition two drives due to bug after bug in Disk Utility, now a brain-damaged version of its former self. But once done, presumably Apple has not yet destroyed the low-level driver code. The intern who wrote the steaming pile of dung that is OS X El Crapitan Disk Utility ought to be fired, along with his manager and his manager’s manager.