it's not that it's hard to play her, but she has a very high skill ceiling (and playing her at her highest potential is mechanically pretty hard, as far as this game goes)
Pick tryndamere for example; the difference between a tryndamere main and someone who randomly picked tryndamere is mainly going to be match-up knowledge tbh. Riven has much more into her; someone who just picked up riven will not only fail in the match-up / knowledge / etc department, but mechanically they really don't come close.
that said, she's very forgiving to play with and even when played at mediocre levels she can still be really strong
easy to do well if her, hard to really master her, and the difference between a mediocre riven and an amazing riven is absurd
meanwhile someone like Malphite, there's a lot less of a stark difference between the casuals and mains.