Untapped Wilderness is simply 'better' than Orion's Grave, because Untapped Wilderness is in Warpath, a faction that is designed almost entirely around ramping resources and playing big high cost characters. Every faction has it's unique strengths, Verore has cheap kill spells, Flame Dawn has fast attackers, Genesis has lots of utility, etc. If all factions had the same strengths the game would become very one sided. The only reason a faction like Genesis would need ramp, is to get Omnitron out and that's what Orion's Grave can help with.
Unlike Omnimind and Shield Generator, Orion's Grave is not a direct threat. Losing your Orion's Grave sounds bad, but it's very unlikely to happen because your opponent has only a very limited amount of artifact removal available and he sure is not gonna waste it on such a low priority target like Orion's Grave. If your opponent decides to use his removal on Orion's Grave, he made a poor decision, because he will not have any removal available anymore when you drop your Omnimind, etc.
Orion's Grave is not a bad card and most certainly not unplayable in ranked and even though the comparison you made with Untapped Wilderness makes the two cards look unbalanced or unfair, Orion's limitations are there to balance out both factions as a whole. ; )