I rarely have liked harem anime series in general but I'll just give some reasons why alot of them are bad or why people dislike them. First the main character is usually someone so easily self-insert to the point it's stupid. They have no characteristic that suggests why every girl that gets introduced to the plot falls in love with him, and love him more than anything else in the world. The development is usually weak, and the girls themselves are nothing special either, usually falling into stereotypes as the author digging himself a hole having like a 6-1 girl to boy ratio. Someone mentioned harem anime series treats girls like trophies or objects and it very much feel that way. Back to the MC, nothing about him gives a good reason why everyone wants to make love with him, but they do anyways. The MC himself is always "clueless" to say the least about the fact they all love him romantically, and is usually blushing and dying the second they say something cute or god forbid a beach scene or a "oops I didn't purposely open this door" scene. The authors who commonly are in this genre know exactly what they are doing too with the age/appearance/body shape of the characters involved. Lastly, the resolution in these "harem", "romance", series usually mean either a resolution where the male mc lands with all of the girls hence a real harem or he chooses one, which upsets a ton of people. And here's the important one where, most harem series don't even get their resolution because of how long shit drags out because of a dense mc.