Failed transfers are very rare. Our analytics show a verification failure rate of 1 in 1300 transfers, meaning 1 file in a total of 1300 disk transfers didn't pass verification of the copy process.
If copy or verification errors occur more than once on the same Source or Destination, chances are your storage media is dying. Especially verification failure is an indicator for this, as OS X catches and retries a lot of read/write errors under the hood. Replace the faulty disks immediately, label with 'do not use' and keep them safe in storage.
Are you getting random failures? Heat issues are often the culprit. Don't pile your disks or card readers. Not helping? Import one disk at a time to keep your gear from overheating.