The reason Zidane & Pep hit the ground running was largely down to how well they knew their respective clubs, which is why I'm an advocate of Terry, Lamps or Drogba joining the club in a managerial capacity in the near future. Emery was brought in by PSG to win or get to the latter stages of a competition he had never done well in, so the expectations were well above his pay grade from the start.
The board/Roman are now just appointing "Yes men" and they are the ones who get the chop when things go south as a result. Only manager that seemed to have full control during his tenure was Mourinho when he first joined. I've been saying for long enough that the structure of the board needs to change immediately if we want long term success instead of what we've been experiencing over the last few years. The club doesn't seem to have any plan or identity which is detrimental.