This rule would make hundreds of cards unplayable, including everything from goblin electromancer to omniscience. I would argue that they need to do away with mechanics that give you free spells, like storm, affinity, dredge, etc. Balancing a few cards that require you to jump through hoops for a free spell is much less of a problem.
Flash This spell costs less to cast if you control the greatest number of permanents among players. Shroud, indestructible At the beginning of each player’s upkeep, for each permanent type, if that player controls the fewest number of that type of permanent among players or is tied for fewest, the player who controls the greatest number of that type of permanent gains control of each permanent of that type that player controls. Creatures gained control of this way gain haste until end of turn.