The Problem:The problem, as I have always had with Metal Wonder, is that when it's good, it encourages one player to not play the game at all, in order to not take unnecessary risks with their idols. On a mechanical level, what makes something like Omen of Damnation acceptable, is that Omen simply punishes threat generation, but doesn't put the game out of reach, because the generated threats are still threats.
Wonder on the other hand, hits the two fastest Scroll types- spells and enchantments, and puts games against decks that rely on them into less of a situation where they need to counter it, and more of a situation where doing anything (even destroying Wonder, especially in multiples) results in unnecessary turtling and awkwardness- because sitting on multiple Wonders against the right deck is just "oops, I win."
My Solution:
Still desirable to deal with for spell/enchantment heavy decks, fills a design hole in Energy's need for supportive structures, gives Energy it's first access to Sift (something the resource desperately needs a moderate amount of.) But most importantly, it doesn't punish anybody for playing the game, it just allows it's controller to push the game ahead easier through selection, while still being a solid wall (and importantly, a 3E unit instead of a 4E unit, which raises it's baseline from "useless" in many matchups as well.)