Finally somewhere I can vent, I dislike harems, not all of them I have found one or two I like, but only cause you expect the MC to be the kind of person who can't stick to one lady and world building allows it and the harem members can be seeing doing so. The other is where the MC treats all the harem members the same or isn't really too attached to them and they are unlikely to come back. I can usually bare most harems since I go in expecting the harems and know what I am for.
However there are certain things in harems that just ruin a story for-
1) The MC guy and MC girl are clearly done to make them out to be really special to each other where if the MC has to pick he will pick that girl. Going to the point where they are literally soulmates and in love, but then have a harem despite the fact the guy has his own true love.
2) The commonly called unwanted harem troupe where the character in question doesn't want a harem or either is not the kind of person to make a girl have harem, but is forced into it anyway.
3) All harem members don't have a romantic relationship with the MC, nor any real feelings, but they all get thrown into a harem at the very end just for the sake of it.
4) Agreeing to share the MC goes against the characterisation, where girls have shown to be the jealous type or they are simply not the type to share, but it ends up happen anyway.
5) And the worse is when a story does not start off as harem for example it starts off with this serious, mature and dark story with some drama and emotional moments usually with a main girl that get you hooked only to be ruined in the end. The MC does interact with girls, but it is clearly as friends/comrades and nothing in the romantic way while the girls end up liking the MC in a sense, but they won't throw themselves at him nor push him to like them and be alright if he has someone and have their own lives. However then it would suddenly start throwing in comedic sexual interactions, that boarder creepy. Both the MC and the girls in question progressively start becoming retarded. This keeps going and till the good plot points that got you hooked to the story in the first place are long forgotten and then in the end despite no romantic or anything being sorted out the MC suddenly gets with all the girls despite never having been more then just friends for the whole story.
If it's going to be a harem make it clear from the start and make it relative to the plot. Do not write an amazing story and plot only to force in a harem that has no place and it is not what the readers are expecting after getting hooked to the story.