I have been in charge of several integrations, my company policy is to keep the owner in the General Manager role at least two years to ensure there is a smooth transition. Sometimes this is not the case. Earn-outs related to short term financial performance usually crate tensions that work against the deal.
I think that the more different the business model and processes of the acquired company are to the buyer, the more you need the previous owner (in my case now they were minority shareholders) to serve as bridge between the two organisations. Also you need somebody from the buyer to act as a bridge to ensure both sides are aligned.
Empathy is a must, mainly when you acquire companies from different countries and cultures. The selection of the integration team is fundamental for success. The wrong team can kill a excellent future.