Is 
viable on assassin mid? It can take care of 
, 
, slow, stun,... all the mortal enemies of any assassin imaginable. Swap is out with 
on someone like Zed with high mobility shouldn't be much of a problem right?
Terrible.
Especially on Zed. Flash adds so much potential to his kit it's unreal. Just buy a QSS which is cleanse 2.0 on a smaller CD with high damage once it's Scimitar.
Cleanse really is just like a buzzing house fly. Fucking annoying when you encounter one but relatively insignificant in your daily life. You'll escape some situations, but honestly flash will get you out of more/get you in more.
