It can't reduce colored mana requirements
They are functionally the same card, but they might have been washed, or played a lot without sleeves, or someone picked it up off a table and put fingernail marks on the edges
If you flip a second Illuna, you can add it to hand and then mutate it onto the first one for a double trigger (and no legend rule!) Its harder to see exactly what to do with Vadrok, Apex of Thunder , since one of the best spells in Jeskai to recast Deafening Clarion will also kill Vadrok unless you mutate it under something with better stats
Rather than simply a cheap crossover, MTGs designers went all out in making sure these decks were worthy of the Warhammer name
Whether this deck ends up as Gruul, Golgari, or Jund, I expect players to test it out at least
Honorable Mention 1 You're probably playing Lands