Ok so I of all people am not gonna bash harems, I like them myself. It's just nowadays there's problems with people making harems:

Rules
Nothing wrong with rules in a Harem, some are understandable. Like if you want to join, prove yourself, especially if it's an enemies to lovers situation. Sometimes, the rules can seem borderline controlling though. Like I was reading a story (which I won't name) and a girl said they're only allowed to have sex with the mc or each other if the mc gives them permission. There's also the rule that they have to be ok with him having sex with other girls not in the harem because it's just sex not love. This seems toxic in my opinion, but to each their own.

Sizes
My LORDS ABOVE people don't know what sensible means on this app. I remember some 2-3 years ago I read a Union/Crossover story so I'm thinking "Oh, cool" but there was mostly Harem pages. This wouldn't be bad if there wasn't about 21 FUCKING PAGES dedicated to his harem. Oh and another author that just copies and pastes the same Harem list with almost every girl from like 7-10 different series and puts "And More" at the end of almost half of those franchises There's nothing wrong with a large harem but there is such thing as TOO BIG.

Alpha of the Harem
Now look this line is so fucking overplayed it's not even funny. If you don't know, some authors just get REALLY fucking lazy and have a girl the mc dates say "You can have a harem, but I'm the alpha" or something stupid like that. Why is it lazy? Because it proves that you don't put any thought into writing a decent harem, meaning you don't try to explain anything and try to do some kind of submissive fantasy or some shit like that. Now if it's like your gf is like "You can fuck girls, but I have to watch/join" or something REALISTIC, then that works. 

Lust
Didn't know what to call this but I sometimes get annoyed when the author makes the girls throw themselves at the mc with no rhyme or reason. Like if a handsome guy went to a new town and every woman there just swooned for him without a second thought that's lazy writing. Now if it's something like a DxD story and the girls went crazy because he was an Incubus, that would make more sense. What I'm saying is without execution, your story seems lazy. It isn't like this ALL the time, but a lot of people aren't good at execution.

Density
We knew this was coming. Now, there's nothing wrong with making a character pretty oblivious to girls that like him, it's kinda funny actually. But as we all know, some authors like to take this up to 11 and make the mc borderline retarded when it comes to girls showing signs. Like yes, many women nowadays don't give good hints, but if a girl is literally SITTING on your lap and making you look at her and you can't tell she wants you then you're just......I can't think of anything to say.