Every add-on acquisition comes with a synergy case, usually built around some combination of cost savings from consolidating overlapping functions and revenue upside from cross-selling into combined customer bases. 

Both categories of synergy are real and achievable in principle. Both are also, in the specific cases that don’t survive contact with reality, built on an assumption that the synergy will materialise largely on its own, once the deal closes, rather than requiring genuine, dedicated execution effort that competes for the same limited management attention as everything else already in motion. 

The cost synergy case tends to fail when the “overlapping functions” being consolidated turn out to be less overlapping in practice than they appeared on a slide. Two finance teams that both do “finance” often turn out to have meaningfully different systems, different processes, and different institutional knowledge that takes considerably longer to consolidate cleanly than the org chart comparison suggested, and the interim period of running two parallel processes while consolidation happens often costs more, in time and confusion, than the synergy case accounted for. 

The revenue synergy case, as covered in more depth elsewhere, tends to fail for a related reason: cross-sell is a sales motion that has to be actively built and trained into a team, not a natural consequence of two customer bases now technically sitting under one roof. Absent dedicated effort to build that motion specifically, the two customer bases tend to remain largely separate in practice for far longer than the synergy case assumed. 

The pattern underneath both failure modes is the same: synergy cases that treat the synergy as something that happens once the deal closes, rather than something that has to be actively built afterward with dedicated ownership and real time allocated to it, are the ones that don’t survive contact with reality. 

Synergy cases that explicitly name who will build the synergy, what specifically that building looks like month by month, and what it will cost in management time to actually execute, are considerably more likely to deliver something close to what the original case promised.