In my thirty-year career I went from being dragged in as a subject matter expert for consultants working for my company, to the person who directed them. My overall feel is that about 25% of those engagements were very useful, 25% somewhat useful, and 50% net negative.
McKinsey (who I worked with rather a lot because they had the ear of every fucking executive at my company) lowered the success ratio. I worked with them (either directing or as a principal SME) maybe a dozen times, and exactly one of those times went great, though I had to corral them into making that one work. The rest were just, as others have said, them recapitulating whatever the interviewees said, while also trying to upsell upsell upsell. Got, I hated them, especially knowing that those little corporate quislings were reporting to senior management on me.
Since there are so many examples of bad outcomes in these discussions, I’ll give you a good one. We decided to merge two large internal organizations (one coming in via acquisition) in our company, and we put someone internal in charge of integrating their IT platforms. What a neverending clusterfuck. After 18 months of wasting time and money, we hired in PWC to manage the integration, plus a boutique consultant to manage some of the tricker bits, and it worked well.
ETA I did just think of something else. Twice in my career, management consultants were brought in to re-evaluate decisions I’d made, because some executive didn’t like something I’d done. To their credit, both times the (two different) consultants validated what I’d done, though one didn’t like how we’d done some implementation. So, consultants can very much be venal, but not always.