Tim said it best. The front/back combos are the real "Monster" of T206.
I found the Polar Bear backs subset, which is "only" 250 cards, to be much more of a challenge than the 520 card typical set, even though Polar Bears are only slightly scarce. At one point I was really questioning if McGraw Portrait with Cap really existed with a PB reverse (it does).
Another challenge of this "Monster" is to attempt a subset of all the backs for a given front, especially if it includes Drums, Uzits, or BL460's! Jamie completed his regular set in 6 months. But ask him how long he's been working on the Seymour Throwing subset
Best Regards & Happy Collecting
Craig