And I'm pretty sure quicksand has been nerfed from its earlier versions. I got more kills from rain of stone lagging than quicksand preventing fleeing. Even ordinary, no-legacy people with heavy carry weights could escape it quite easily.
I don't know how Onya gets so many kills without rain of stone. That spell is amazing. Much better to take earth over water. Engulf is nice, but so is iceshards, forked lightning, controlled fireball... Engulf is better than those, but not by much.
I'll say one thing though, a prepped invoker can make just about anything run. It's not going to kill skilled players, but it will make them run.