It could return virtually every non-land permanent an opponent controlled in one fell swoop (and there are stories of people using Aurora Griffin to return the lands as well!), turning a stalemated game into an instant-win with your hordes now free to march unopposed through enemy territory
"When you cast a spell with convoke, tap your army to make it cheapereach creature pays either or one mana of its color." Official Rules Reference From the Magic: The Gathering Comprehensive Rules Section 702.51 702.51 Convoke KEYWORD ABILITY Official rule text from the Magic: The Gathering Comprehensive Rules document
Typically Mentor is better in aggressive Boros decks or tricksy tempo cawblade style Azorius decks
First, most overpowered cards are still overpowered when they're restricted, so, in formats where there are only a few restricted cards, all that restricting overpowered cards serves is to add randomness to the game
Dark Depths was broken by a certain two-drop named Vampire Hexmage
Sun Titan itself is a monster of a card, and Sun Titan into Flickerwhisp takes me back to an earlier time of glory
