Buffing your whole board for two mana is pretty strong
Kindred Charge For Kindred Charge to be good, you need creatures that share a creature type
Destroy all creatures with power three or greater
(Right now, the most dominant cards in the format have a level of dominance that is unheard of, even including all of the other broken decks and banned cards we've been talking about
There are actually astonishingly few threats in the current Modern metagame which survive Unholy Heat, and a lot of play patterns are defined by the existence of this card
Brainstorm Ponder Expressive Iteration Fevered Visions Jori En, Ruin Diver Rielle, the Everwise Firemind's Research Yusri, Fortune's Flame Jhoira, Weatherlight Captian Kraum, Ludevic's Opus Niv-Mizzet, the Firemind The Royal Scions Ral, Izzet Viceroy Squee's Revenge Aegar, the Freezing Flame Rhystic Study Counterspells with a hint of Redirection It's not blue without a counterspell or two