In my opinion, which has no research regarding sell prices, etc... They are less valuable for resale purposes because they move most of the "key" label info to the back. If I personally had the choice when buying an already slabbed/graded book: Between a custom label and a universal of the same grade, I'd pay a little more for the universal. (A LITTLE MORE)
That being said, for my own collection of books I'm slabbing with no intention to sell, I sometimes go with the custom labels because I like the look.
So: Contradictory and unhelpful. Take it as you may.