Buy-and-build strategies get modelled on synergy and revenue growth, and both of those things are real and achievable. What rarely makes it into the IC memo with any specificity is the actual cost, in management attention and organisational disruption, of executing the integration itself, and that cost is usually the thing that determines whether year one goes well or badly far more than the strategic logic does. 

The specific cost that gets underweighted almost every time is senior management bandwidth. Integrating an acquisition properly, aligning systems, aligning culture, aligning go-to-market, requires serious time from the same small group of senior people who are also meant to be running the existing business and hitting the plan that was already in motion before the acquisition happened. That bandwidth isn’t free just because it doesn’t show up as a line item, and a management team stretched thin across running the core business and integrating an acquisition simultaneously tends to do both worse than either done well on its own. 

The second underweighted cost is the disruption to the acquired company’s own momentum during integration. A business that was performing well pre-acquisition, which is usually why it was an attractive target in the first place, often slows down during the integration period itself, as key people deal with uncertainty about their role, systems get disrupted, and customer-facing processes get temporarily degraded while things get combined. This dip is common enough that it should be expected and planned for, but it’s rarely built into the year-one model with any honesty, because acknowledging it upfront makes the deal look less immediately accretive. 

The practical fix, before any add-on gets approved: model a genuine integration cost in management bandwidth terms, not just financial terms, and be honest in the plan about a likely temporary dip in the acquired business’s own performance during the integration window. Deals that account for this upfront tend to manage through year one calmly.

Deals that assume synergy arrives smoothly and immediately tend to spend year one explaining to the board why it hasn’t.