I don't think "ultra rare" and "not desirable" overlap, specifically because an ultra-rare card does not transact enough to make a factual assessment of desirability. If a card or set has enough of a sale history to be labeled as not desirable, it isn't ultra rare.
Beyond semantics, so much of collecting outside of the mainstream depends on having a feel for the value rather than hard data and acting on it. As for value and demand, frankly, it is largely dependent on how well you can hype the rare item when you want to resell it. I'd venture to guess that many collectors simply do not know what an R94 is, let alone that there are two Babe Ruth cards in the set, so the only way they get to perceive it as desirable is to see and hear about it.
The only thing I know for sure is that whatever I think of an item's desirability, it is more likely than not that I will be surprised when it finally sells. I was floored by the price of a lower grade 1962 Pittsburgh Exhibit Sandy Koufax in the last REA. No idea it was at that level of demand, but it is a very tough card with no recent history.