It is the single overriding factor in determining success. On the other hand, if a solution is built while in continual dialog with its human interfaces so that the solution is also their solution, then it will matter little how good it is. They will make it work. Several things are at play here. What I have is my perception of the user's perception.
Perceptions are fluid.
They change over time and from person to person. One must stay engaged as the problem is a moving target. This is a human dimension. What is really being talked about here is expectations, unmet human expectations. Rarely can expectations be quantified.