I think consultants have a role in a many areas in M&A. Specifically in the DD, I’ve seen (and support) consultants being retained to handle valuations (or at least sense check them) working with company teams. I think they can be critical to this and other areas of DD – mostly because they can (should) provide a third party independent assessment to ensure the in house team isn’t overvaluing or making overly positive assumptions because they want the deal to go through.
I’ve also seen them used for integrations but I think this can have more pitfalls. The idea is always that they work with a in-house employees to deliver, but generally, I’ve seen this go bad two ways – one, the inhouse team ends up doing everything themselves as the consultants aren’t adding value and then they are just an expensive roadblock and two, the consultant ends up doing everything because the in-house team is busy with day to day operations but then post integration they do not understand what was done and why decisions were made. Consultants can be very valueable in these but only if used correctly (much like everything else I guess)