Not hired one directly but I joined a team during the transition from agency to in-house. The transition took 3 months for us to be self-supported as the platform came with zero documentation.

Due to politics the MVP had 2 false starts at other agencies before the third company built it in a crazy short timeframe. The third company was only going to built Android/iOS apps to an API specification provided to them, but ended up doing the entire backend too.

The false starts were down to lack of clear guidance as to _what_ the client wanted from the agency and very slow feedback loops regarding progress. The first attempt was absolute overkill (and very slow velocity), the second attempt was nothing like the intended idea and development stopped after 12 months. Runway was getting shorter so third agency built it.

The biggest mistake was the client hiring a fresh junior as their first in-house developer, and them not having sufficient experience to identify parts of the platform that would become obvious scaling problems. Later on these became very expensive problems!!