Four mana is a massive cost, a creature has to be Titan levels of good or you have to have vast amounts of spare mana before you are paying it
Plus, every time your dragons deal damage, you get to dig through your deck and even bounce dragons back to your hand to dodge removal or retrigger enter-the-battlefield effects
I never actually figured out a time to bring it in, so it would probably be better as Imprisoned in the Moon as another way of dealing with planeswalkers, or even just more counterspells
Rebels was very unpopular (and a bit overpowered) the first time we did them, so I'm not optimistic of their return, even this new graveyard version
Youll be able to flood the board with a lot of humans and simply overrun your opponents
Sometimes it crops up in Modern in Grinding Station or Egg-style strategies as the cards are aggressively costed, you can recast them with Underworld Breach in play