Expert take
Vex is an excellent choice if you want to punish players who confuse mobility with safety. She requires less pure mechanics than an assassin, but much more discipline than she seems to: holding Doom, choosing her angle, not recasting R by reflex, and converting kills into objectives. She is not ideal in every draft, especially when enemy range prevents her from stepping forward or when your team cannot follow. But in the right conditions, she makes every enemy dash dangerous and turns mid-game fights into traps that are very hard to cross.