While I do like the level of skills that Earth has, I feel that it's simply not good enough to compete with other elements. Darkness has card denial, upgrade-boosters, salvaging bonuses, teleporting skills, advanced ways to gain exp, and other unique ways to tackle the enemy.
Earth, on the other hand, does not. Most of their abilities lie in being heavily defensive, or getting electrum. They have no card denial, teleportation (heck, even their movement isn't something to talk about), unique exp boosters, and their ability to create Elite Shriekers is similar by means to the ability to use Giant Frogs. I feel that Earth simply doesn't have enough combat capabilities to uniquely gain an advantage on the opponent.