Injustice: Gods Among Us is pretty great. But, the Wii U version is missing a feature found in the PlayStation 3 and Xbox 360 counterparts. Namely, the iOS game is meant to unlock content in the main game, but that feature is curiously missing in the Wii U version.
Siliconera noticed the missing feature in its review copy. This has precedent on the Wii U. The port of Mass Effect 3, for example, didn't link up to its companion app like the other console versions.
The Wii U version does have a few unique features of its own, like the ability to play on the GamePad or map a move list for quick reference. But if you've been playing the free-to-play iOS game to unlock extras, the PS3 and 360 are the go-to platforms of choice.