To be fair to the band, they have put out some decent archival releases. Archive 1 and 2 had plenty of unreleased or hard to find stuff. The boxed sets compiled all the main tracks, had a more complete Rainbow concert, and had interviews for every album. That's pretty good. A number of artists haven't done this much.
I think it becomes frustrating when we compare what has been released to what we know exists for possible release and when we compare to other artists that have gone into their archives more thoroughly.