Okay, so I've seen a lot of these kinds of fics pop up over the last few years, and honestly? The way they handle dynamics is a total mixed bag. Some authors just pile on the love interests without any real thought, turning Izuku into this weird wish-fulfillment magnet where every girl in Class 1-A suddenly drops her own personality to orbit him. It gets boring fast.
But the ones that actually work, for me at least, are the ones that treat the harem as a problem to solve. Like, a fic I read recently had Izuku absolutely panicking because Uraraka, Asui, and Jirou all asked him out in the same week, and his analytical brain short-circuited trying to figure out the 'most heroic' way to handle it without hurting anyone. It became less about romance and more about his core conflict—wanting to save everyone, even from emotional pain. That felt way more in-character than him just blushing and accepting it. Those dynamics explore his anxiety and over-preparation in a new, surprisingly stressful context.
The worst ones skip the character stuff entirely and just jump to fluffy domestic scenes or, ugh, lemon stuff. Misses the point completely. If you're gonna do a harem, use it to poke at the cast's personalities, not erase them.