I worked for a McKinsey competitor (Boston Consulting Group) and I also worked with McKinsey as a customer. They are just like a virus, they have two goals:
1) Self Preservation
2) Procreation

They infect the nucleus (management) and make copies of themselves (bring in more consultants).

They have various tactics:
1) Interfere in the relationship between management and their people
2) Singular focus on optics, not results
3) Recruit at virus breeding grounds - elite universities where most of the graduates have been bred to think they’re smarter than everybody else by an order of magnitude
4) Work ridiculously long hours at key moments and change direction in the middle of the nite and tell nobody but management

Too bad, they’re smart and have access to good intel. But the poison they bring with them isn’t worth it. However I have a method to extract value from them and reduce their negative impact (when I’m managing them):
1) Give them a very clear charter and make it clear what boundary they shouldn’t cross
2) Require then to work with the company’s team collaboratively (and check in with your team)
3) If they violate either of the above two, fire them immediately…zero tolerance.