Great post but I just wanted to throw in my two cents. My recent dealings with Accenture were subpar and the firm wasted ~$2mm on a project they worked on. I would imagine what Disney hired them for they were better suited to accomplish. Essentially Accenture couldn't properly grasp our existing data matrix used by the firm (large acquisitions made systems a complete mess) which extended the time of the project significantly and their services were terminated for the project.
They've had some amazingly successful projects, some amazingly unsuccessful projects (like why they changed their name) , and everything in-between. Same for all of the big guns in that space - EDS, InfoSys, etc.
At the base level, it's still a Buyer/Seller relationship. They're selling man hours/man days, and man years - bodies, along with some methodology. The buyer is looking for expertise.
But there are so many ways a well defined project can be doomed from the start - on both sides and at all levels. I've seen large, well-defined projects doomed from the start where everything is set up perfectly, then a C-level decides it's "too expensive", so 30% or more of design time, or testing, or project mgt, is cut out during negotiations.
Then there's the quality of the Team. They have thousands of consultants they can bring to the table. But, like all corporations, there are a few hundred that are truly the super stars, the architects. So, projects can be affected by what other projects they have ongoing.
Think of it like hiring the best, and largest paint company in your town to paint your house. It's a 3 grand project. If that same company has a 50 grand project to paint the new Lowe's Hardware, and another 60 grand contract with the State happening at the same time--- can you guess where all of their master painters are going to be working? But, they're still not turning your project down.....