Their stuff won't solve problems. It only compounds existing ones. It behooves you to engage in a critical assessment of the veracity of promises made and the overall impact on the work to be done. Whether you're changing processes, systems or people, the impact will affect all three. It's impossible to change a process without changing the dynamics of the people charged with implementation.
People move processes and systems.
The same is true if changing the work habits or attitudes of people, processes and systems will be affected. It's up to your consultant to understand the domino effect throughout the organization. Buyer, beware, whether you're negotiating with one of the Big Six or an individual. In a recent case, one of the Big Six consulting firms was hired to install a more efficient computer system for a large energy firm in New England. From the get-go, the system and the people behind the system were in over their heads.