I think the harem genre has to be the most often fucked up of all genres, at least ones that come to mind. 90% of harems don't even have a likeable main character. I think the basic building blocks required for a decent harem are:
-Likeable MC, No betas, no dense retards.
-Reasons why the girls like the MC
-Not a fucking Japanese high school setting, settings in the past are ideal because polygamy can be possible
-Variety of girls each unique, there is no need for 2-3 tsunderes
-SLOW reasonable relationship development. If the girl wants to fuck the MC after a week there's a problem
-No obnoxious bickering from the girls
I can only think of a handful of harems that don't horribly fuck something up.