The flashback is a self-replacement effect meaning that the card will be removed from the game before buyback would return it to your hand
You dont need five treasures to transform, but rather five artifacts
It will excel in some decks more than others, that much is true, but where itll shine the most is in builds that want to do fight spells and deathtouch, or mess with dealing damage and using damage redirect effects
Ajani, Caller of the Pride Akroma's Memorial Chandra, the Firebrand Elderscale Wurm Garruk, Primal Hunter Jace, Memory Adept Liliana of the Dark Realms Nicol Bolas, Planeswalker Omniscience Primordial Hydra Serra Avatar Sublime Archangel Thundermaw Hellkite Vampire Nocturnus Worldfire
Azorius Omniscience also features a transformative sideboard plan, capable of swapping out its combo elements for a creature-heavy configuration
The third mode is often relevant in the face of good 1-drops or tokens