The problem with shards is that they dominate and overshadow all remaining cards. The process of deckbuilding becomes deciding which shard set to use against which shard set, narrowing deck options and making deckbuilding too boring. Also, they are extremely hard counters. For instance, most if not all rushes will almost 100% lose against a set of sosacs, no matter what other cards the deck has. You may be terrible deckbuilder and a bad player but your sosacs will win the game for you. The air one will counter all shield decks etc etc. Shards are too broad spectrum counters, steamrolling tons of decks without much need of deckbuilding&smart playing and taking away the fun of actually having some chances.