I think a major reason why people had such a problem with Rhys in ACOFAS is because he was and is idolized so much. A vast majority of the fandom treats him as this amazing male specimen that can do and has done no wrong– the perfect roll model for what you should look for in a man. So then when ACOFAS came around and people noticed character flaws, it was a shitstorm. (”What do you mean my fave isn’t perfect??” “You’re not allowed to stan Rhys anymore!” “I can’t believe Sarah wrote him so OOC!”).
Rhysand is not “the perfect male”. He makes mistakes, he does things in ways that can be harmful to other characters. Yes, he absolutely has good qualities, but these same qualities can very easily be turned around on their bad end. Which is just good characterization and writing.