Harem come in many shape and sizes>harem hate each other>harem like each other>harem compete with each other>harem don't acknowledge each other>harem isn't threatened by other women because she thinks mc will chose her.
ultimately it's all shit though, correct me if i'm wrong thought i thought NTR was more about cheating and rape, harem is more like no of the characters are in a serious relationship and are cursed by the author to only have eyes for the MC who is an indecisive japanese man.
What i mean to say this is NTR as far as i understand it is made to make people sad, and characters are sad after being ntred, where as women rarely show any negative emotion pass being tsundere or very rarely yandere towards the protag.
tldr: it's not really right to compare them because the tropes are executed far too differently i think.
NTR being more realistic and harem being more fantasy.
TLDR:TLDR: Harems are shit and so is NTR