He would have tried to turn him into a dink 'n dunk passer, instead of letting him be the downfield passer that he thrived at being.
No quarterback thrived under Rees' tutelage, except for Ian Book, and I actually credit Book for succeeding despite Rees being his coach. In 2019, there was a stretch of time where Rees tried to force Book to be a carbon copy of what he was in 2013, and it was no surprise that the offense was stifled. Once he let him be his usual gunslinger self, those problems magically disappeared.
McCarthy did a fine job in 2023. While Roman Wilson had a good season, including 12 TD catches, he wasn't exactly the spitting image of what you'd expect out of a WR1, and McCarthy managed to put up some solid numbers despite not having that dominant WR1.