While we are on the topic of listing problems with consultants/outsourcing - another potential problem is the A team/ B team switch. During initial engagement you are often talking to somebody who appears to actually know their stuff - you hire them and find that this knowledgable person has moved on to the next big sales, and you've got the B or C team.
What does happen is that you were pitched/sold by a perfect PowerPoint presented by a senior partner/manager, but all the work afterwards is done by the 26-year old fresh-out-of-MBA consultants. The partner is still responsible for the project, but mostly on a “I'll review your work later from an expensive restaurant I'll bill the client for”.
They'll also invent the wildest things when it comes to tech work. I was working as an expert on software development and integrations, and often had to tell clients that what the consultants promised is simply not possible.