Here are some lessones learned from my side:
– Change management shouldn’t be just a work stream, it should be the basis for the project management team.
– Employee Surveys should be conducted in all phases of the merger
– The role of leadership/middle management should be highlited from beginning on, you need to push their commitment and engagement during the integration
– All work streams, e.g. IT, HR, Legal need to be always in touch
– I think M&As shouldn’t be organized as waterfall projects, a agile approach, e.g. SCRUM would be much better because of the complexity and dynamics