Beef with Jack Kirby over creator vs company rights to characters and compensation which led to him portraying Big Barda (based on Kirby's wife) in a highly inappropriate and insulting manner in Action Comics. There is also his negative views on immigrants which bled into his depiction of Superman as being "born" in America, being a proud American/Human, caring little for/denouncing Krypton and his heritage.
Oh and he's a transphobe, just learned about that one.