In a vacuum, Clue tokens are much better than Blood tokens (they don't force you to discard, don't need to be tapped)
Heres a sample of Ulalek in action: With your commander in play and 5 mana, cast Herald of Kozilek, pay the , then, while its on the stack, pay the 2 colorless mana
Green and black both have a plethora of creatures in Amonkhet that are greatly undercosted but enter the battlefield with some number of -1/-1 counters
however, one of these reasons was one which many might not have expected
There are also Special Energy cards that give a different kind of boost, depending on your type of Pokmon
Over the last years especially the Moxen, Ancestral Recall and Time Walk went up in price quite significantly