Yes in a lot of cases it may be an afterthought, which obviously would negatively impact the success. But there could be other reasons why mergers may fail even if there is a post-merger integration team working on it. the Daimler Chrysler merger is an example of where integration was planned well in advance but still miserably failed because of how it was managed. The following things are important as well:
1. Not being involved from the pre due diligence and due diligence stage and therefore not being clear about the strategy of the acquiring company or the goals and synergies sought
2. Not having the right team members in the integration team. Doing PMI but with the help of the regular business as usual leaders who may not have time or the competency can also result in failures
3. Not having the right sponsorship from the top leadership can be another factor.