This is probably true if 'client' refers to the decision-maker, rather than the entity paying for the services.
Say you're managing some massive government department, that definitely needs some particular change. You and your team don't have the skills, experience or even desire to do the work. But you can't be seen to do nothing. You can hire a consulting firm to do the work. It's not your money, after all, and your team cannot reasonably be expected to have these very specialist skills. So, you hire the firm. Whether the project succeeds or fails, the firm provided value to you (the person responsible for the decision). In the worst case, you get someone to blame, and your reputation is preserved because: how were you to know this famous firm would not deliver what they promised.
If you use the word 'client' to refer to those who foot the bill (shareholders or taxpayers), then it might not be the case that thes mega consulting firms 'generally provide value to their clients'.
This seems like a large enough instance of a principal-agent problem to be worth studying. And, for government contracts at least, the information is public.
Has anyone done such a study?