Important consideration: If you ever have to recover audio from failed media, WAV will be far easier to recover than flac.
I can salvage raw wav data from file systems. But random pieces of flac data? Yuck! It should be doable but reconstructing from pieces would be a lot harder and more time consuming. So that's a good reason to not use flac on masters. Recent builds of flac have improved but there have been some cases in the past where it wouldn't store all of the wav audio in the flac. So that's another thing to watch out for and doggedly verify.
Fwiw, in the data world, "backup" implies that the original will be kept. Archiving implies that the original will be deleted to save space and the archive is THE copy (which there should be other backups of).
So whenever you discuss archiving, it is important to be clear about that... I would never want to trust a single optical disc to hold the only archive of a show. I don't think multiple copies from the same spindle of disks or batch would be much more reliable. I think having a magnetic copy and an optical copy is important.
One concern is if you lose your hard drive copy (lightning, theft, etc) and suddenly that optical disc is your only copy...