I agree, it is all about clarity and communication. Especially in the early stage and right from the beginning. However, this is the challenging part since many parameters are not clear/require time to define at the beginning.
Nevertheless, the key employees need to be identified in the early stage including the opinion leaders, which are not always the same persons.
What I see as a key factor as well is the “brand” integration process. The loyalty to the brand can be of utmost importance of employees from the acquired company, especially previous owners/founders. Having an open communciation, involvement in decisions and understanding can positively influnce the process of retaining key employees.