Being printed at rare is huge here
Fairness: Prevents "marked cards" in competitive play
However, that five-mana difference materializes through two very real drawbacks: the cumulative upkeep cost of one generic mana, and the ability of your opponents to player it around it
My theory on Sol Ring (and ramp in general, for that matter) is either we want it all the time or we should work around it, because if it means 10% of our deck is dead after our opening hand, it's probably not worth it
6 Umbris, Fear Manifest Formerly the most popular option for Horror-Tribal Commander decks, Umbris, Fear Manifest is a 1/1 Nightmare Horror for five mana that gets +1/+1 for each card its owner's opponents own in exile
Hidden Cove and Rapunzel also add to the Will Power of your characters, which will come in handy so you can keep them around long enough to make an impact