MKX is more balanced than Injustice in terms of how many fighters are worthwhile and viable with most characters having at least one good variation.
Injustice is more balanced when it comes to the amount of playstyles that are viable. Flash went hard as fuck offensively, Zod went just as hard with his zoning, Bane just not giving a fuck, Aquaman with dominant footsies and defensive tools.
It just depends on the type of balance you're looking for. Yes MKX has a smaller gap between your Shinnoks and your Erron Blacks, no true dumpster tier characters while the same can't be said for Injustice. But Injustice represented every playstyle with a powerful character, same can't really be said for MKX. A character loyalist would enjoy MKX way more, if Arrow was your favorite character get ready for pain. But imo seeing two fucked up characters that do vastly different things going at it is better than two balanced characters that do a very similar thing go at it.
Depends on the type of balance you value really.