The implementation will be more than you thought.
It will take more of your time.
And be more complicated.
It will require way more communication.
You will lose a valuable person during the project, and it will force you to adjust.
It will force you to include someone you never considered.
No matter your expectations going in, the implementation will be more or different than what you thought.
It is not the vendor’s fault.
It just is.
This should be your baseline when you start.