The drop rate for 3* rainbow runes/shards creates a huge bottleneck. The drop rate for 3* rainbow evolution runes in the normal game is abysmal, especially now that only one drops when you get it. In the dungeon I've been getting a lot of 3* green shards, but the drop rate for 3* rainbow shards is also really low. Obviously you need significantly more rainbow runes/shards than any other color so having the drop rate so low feels unnecessarily punishing and taking the fun out of the game.