I'm not going to mention anything specific. Overall, I like the game very much, and summer jam was hype as hell- a great illustration of how awesome this game is.
But if I had to say why I'm playing KI now instead, I'd sum it up in a single sentence:
"The meta of Injustice is grating"
And isn't it? It can really wear you down. The game is amazing because every character completely destroys the meta of traditional fighting games in some way- either with an abnormal special ability, setups, or an absurdly good move/string... At the same time, having to deal with all this meta-breaking stuff can really wear you down mentally. Every time you go up against a character, they have some kind of meta-breaking tool that you need to seriously adjust for in a way no other fighting game demands of you. Sure, it doesn't LOOK as crazy and insane like Marvel vs Capcom or certain anime fighters- but mechanically speaking and when it goes down to matchups, it's probably one of the more crazy thing out there. Every character has both obscure gimmicky tech that will destroy you if you aren't aware of how to counter that bill, and non-obscure "you just have to deal with it" traits that make playing against them different than playing against any other character.
I do like Injustice, but the game's meta and matchups can be a bit too much to handle sometimes.
*Edit* Just in case I need to clarify this any more painstakingly: I like this game, but it's tiring to keep up with. *