Although there are characters that have five strength and could successfully challenge Jumba, his willpower is still high enough that he should be able to withstand a head-on challenge
Keeping Kefka alive a whole turn cycle to use his ability again might be tough, but at least in black there's a fair few protection spells
Additionally, using it early in the game, if your opponent has no energy in hand, can quickly give you an advantage
However, this could work very efficiently against Fires of Invention decks that have been dominating Standard for some time now
For its base cost casting of four mana, this is still an okay card, especially as it's potentially giving you six mana worth of creatures back, but its sneak cost is just two mana , which feels slightly broken, especially as it also effectively turns this sorcery into an instant
While that's a question only you can answer, I'd point most players towards Bracket 2 or Bracket 3 to start