Apart from the fundamental flaw of its premise, as you mentioned, my other big problem with Injustice is that Superman constantly gets his entire ass kicked. Wonder Woman fucks him up, Hawkman fucks him up, the Lanterns fuck him up, Black Canary fucks him up, hell even Alfred fucks him up.
We're led to believe that a Lawful Evil Superman would be the end of the world, but really the only reason he was a threat at all is because the plot required him to constantly get rescued by OOC JL members, or spared by the people who obliterate him in battle. Honestly, he should have kept his Yellow Ring and stayed overpowered as fuck. Maybe then I'd buy him as a universal threat, but nah, they put him back in regular Superman mode and let his ass get kicked like four more times before the ending. His design as a Yellow Lantern was also immaculate.
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(He/Him) I write Naruto Fanfiction. But I am definitely not a furry.