For starters - 'McKinsey' actually doesn't do implementations. So you're not going to hire them to build something. Second, it's hard to fathom their value because it's very secretive. There's nary any real objective data.
Oh looks like they've changed their model. I would trust them to give strategic advice about something, or maybe some insights from an industry expert they might happen to have. Actionable or not, who knows. But I wouldn't trust them to 'build' anything.
I worked in McKinsey Digital. Here's an example of the typical type of project we did:
- Client is a legacy bank
- Client wants to be a fancy new cool digital bank like Revolut
- Client brings in McKinsey to help get them there
- McKinsey (traditional side) does a strategy engagement first (4-8 weeks) to define priorities, budget, etc
- McKinsey Digital + traditional consultants come in to do the implementation. This includes architecture, actual tech building (more on this below), assisting with hiring and HR, coaching, financials, integration/de-integration with rest of business. Essentially everything involved with starting a "new" business.
If you want a single company to come in and literally do everything to build and run a new business I do think you would need an MBB-type company as they can bring in all these different skills (business, people, tech, process etc). I'm very reluctant to praise my former employer.
On the actual tech building side, McKinsey would typically bring only 1-3 technical people as it's cost prohibitive for the client (I was one of those people). McKinsey would help the client hire new people or bring in contractors (i.e. Nagarro and the likes).