Most teams in organizations are slow teams. They go into self-doubt and risk-aversion when uncertainty exceeds their threshold. They become more unaligned and unresponsive to change. Mergers and acquisitons are hotbeds of uncertainty. Slow teams get paralyzed in the process.
Organizations that don't know how to make organic growth possible don't know how to help slow teams become well-performing nimble teams. The only way the merger or acquisiton could work is if the participating organizations help teams move from slow to nimble. This takes a complete rethinking of culture. We now know how to make this rethinking possible.