First the two caches that I mentioned are not multi caches because the owner only has one waypoint all the caches you must get are someone else’s. Second difficulty and terrain ratings are interrupted values. They have no solid rules only suggested guide lines. I cannot compare a cache like Washington Delorme Challenge GCQQ9B, and Washington County Challenge GCZB73 with a level 5 difficulty cache. The level 5 could be because the owner thinks his puzzle is hard or that he has made a really good container that will be hard to find and I think with a little time you can come up with more reasons that someone would call their cache a level 5. The quest cache would have criteria for being a quest.
quest (kw st)
n. An expedition undertaken in medieval romance by a knight in order to perform a prescribed feat.
I think that caches like WDC fit that definition. They take multiple days, extensive resources, many hours of planning and a thirst for adventure that is above and beyond a normal geocacher(if there is one).
And my opinion for adding it as an attribute is that attributes are not cherished like a find icon or a geocoin icon. When have you ever heard anyone say I have three cache finds that have a tick attribute or a stealth required attribute. These caches are a special part of geocaching that like a mega event not everyone will be able to do but if or when they do it will be something they will remember.
I really like your statement it makes me think more in-depth about this subject. And I think that you do have a valid point and I have reasons to disagree.