Throughout my career I've spent many years working on both sides of a consulting relationship. As the consultant and as the client, I've seen projects go extremely well and others not so well.
There are obviously many factors that contribute to the successes and failures of a project. Overall, the natural tendency may be to conclude that a successful project was due to strong consultants, while an unsuccessful project was the result of poorly performing consultants. In my experience, however, the lead client or client team can impact the outcome of a consulting project as much as, if not more than, the consultants.