The problem with shards was cards like Supernova, Nova, Cremation and Quantum towers. The shards would get out of hand too easily, and also gave these cards power and they did not need. IT required one of 2 things to be done. Overly nerf the shards/the cards that let you use them easily and make shards hardly seen, or make the shards op. Since they were Rare, OP was the obvious direction to go instead since then they would see use. They also had 1 fatal flaw. They were balanced towards rainbows. They had to be since otherwise they would be op. In other words they were often times too cheap to use in a rainbow, and too expensive to use in a mono. Now they can all be modified to be more balanced and still give the desired effect.
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BTW Im aware there were solution to this such as making shards require all of 1 element to be used, but this would have to effect ALL other cards and not just shards, otherwise a whole new element would have to be made.