I did not say "Difficult to balance"
What I said was:
The issue with Vex as a 4 cost melee champion was that in order for her to find success, her build was VERY narrow. If you had artifacts like Zhonyas or Seekers then she could pop off, but otherwise she would dash around and get herself quickly killed doing very little. This meant we had a 4 cost who was close to unplayable unless you had a very narrow set of circumstances be true, which isn't very good if you want the game and set to be flexible. In addition, when she did finally achieve those circumstances, the play against was also pretty infuriating as she could delete backlines in less than 5 seconds, or go infinite with the stacking omnivamp.
It was a shame because I believed in the melee design and wanted it to work. The idea of Vex dashing around the battlefield was a really cool concept. But the reality was it was too narrow and bad for the flow of the game, so we had to make a last minute pivot and turn her into a standard backliner that you could play from a variety of positions.