Thats something that really turned me away from other card games like Yu-Gi-Oh or Magic: The Gathering
Whenever equipped creature deals combat damage to a player, create a 4/4 gold Dragon creature token with flying and a "D&D" watermark and silver border, then roll a d20 (a twenty-sided die)
While we've had dominant decks and cards like Collected Company, Siege Rhino, Pack Rat, and Rally the Ancestors, those formats didn't see huge drops in attendance, and for the most part, by the time the outcry got loud, the problem card was already close to rotation (if you look at Rally and Bant Company, their most dominant times were in the two or three months before they rotated from the format)
The cost reduction applies only to generic mana in the total cost of spells you cast from anywhere other than your hand
Merchant Scroll only gets a blue instant but there are plenty of targets for it