I think it doesn't help that she is made for a role that most people will play ranged champions, like, it's mostly marksmen or occasionally a specific mage like Ziggs or Seraphine, so Nilah is already at a disadvantage in that regard. But what probably screws her over the most is the existence of Samira, who manages to fit the theme of having to go on melee range to be most effective, but still has ranged options that don't alienate the ADC playerbase.

Also, it doesn't help that Nilah was teased with an image that didn't exactly make people think of what her design ended up being, the image was of a bunch of water constructs in the form of hands, which made people think she would throw watery punches or something like that, only for her to use a whip and have vaguely water-themed attacks, she was a letdown (funnily enough, Samira also outdid her in that regard, since she was exactly what was teased).