New Miasma is still garbage
It's only purpose is basically to prevent a nearby enemy champion from escaping. Except if anything is that close to Cass, she's probably already fucked.
The grounding mechanic is borderline useless, because anything it lands on top of will have moved out of it within 0.5 seconds. If you drop it in front of a champion, it will just use it's mobility to jump over it. If you drop it on top of a champion, it will just use it's mobility before it lands.
Since the slow no longer persists after they leave the poison and it has a brief animation lock, it's no longer even usable as an 'I barely caught up and don't have enough range for Q to hit, so I'll drop Miasma to slow you so I can get into Q range.'
This ability makes no sense. It's pretty clear that this was designed to help Cass defend against mobile champions, but the ability does nothing to achieve this. Who thought this would be remotely useful?