What stops Elise's Rappel?

Neither the tooltip nor the wiki page say that anything prevents Elise from using her E, obviously hard CC and silences would but I've also been unable to cast it while being slowed as well which led me to believe all forms of CC would prevent it. I was playing against a Nidalee not to long ago and found out that her spear stopped it as well, presumably because of the Mark as she had no items that would CC me. So I'm a little confused as to what exactly prevents Elise from using Rappel and why it's not made more clear that things outside of hard CC and silences do prevent it.