For me it’s the way Captain Marvel was shoehorned into the MCU without any real development, and existed as what was basically a walking plot hole. Marvel movies have never been perfect but their whole thing was building characters into it over years. By Infinity War we’d been watching these characters for years and we wanted to watch the end of their stories. Then they just plopped Captain Marvel down into the series at the last second, had the existing legacy characters tell us the audience how to feel about her because the writers knew we wouldn’t care organically, made her disappear in Infinity war and most of endgame because they didn’t actually have anything for her to do except be a deus ex machine at the end of Endgame.
It also doesn’t help that Brie Larson just phones it in any time she’s on screen.