Yeah, I think Poppy's W turned out pretty well, she can use it to block important mobility spells, but only one, so if you block the wrong dash on say, Lee Sin, you stop his engage but then he just bails, but if you stop him from getting out you effectively have him trapped for a while.
Cassiopeia on the other hand has an ability that is far more subjective in its use and I think that it is actually a pretty bad ability. Having the minimum range means that if she gets engaged on she can't keep them from running afterwards, and if she needs to not get engaged on she doesn't have the freedom to place the ability where she needs it. Scissor legs (I actually just forgot her name, haven't been playing much lately) just slides around it, Zed just flat out throws a shadow through it and does his thing anyway, Malphite only cares about it if it lands directly on him before he ults, which it shouldn't ever do since his ult has better range. She just really doesn't have anything going for her to stop most of the threats it is designed to deal with.
It's a matter of good ability, wrong character. Cassiopeia needs an immediate answer to divers at closer ranges because she needs to be at those closer ranges to deal her own damage, while someone with longer ranges (Some kind of support I think would be a good fit) would love to have a slowing field that also prevent enhanced mobility.