Most companies call in outside M&A help at the same moment: after a target is identified, sometimes after a term sheet is already signed, when the internal team needs help finishing diligence on a deal that is already, functionally, happening. That is real work and a legitimate reason to bring in outside advisory. It is also the later and cheaper half of what good M&A advisory actually does. The more valuable half happens earlier, when the deal is still a hypothesis, and it is the half most companies skip because there is no obvious trigger that says "call someone now."
The synergy number is the easiest part of the deck to get wrong
Most acquisition decks carry a synergy estimate, and most synergy estimates are built the same way: take the two P&Ls, find the obvious overlaps, and apply a percentage that feels defensible in a room. That number rarely survives contact with integration, because it was built without answering the harder questions underneath it, which processes actually get combined and in what order, which roles change scope or disappear, and whether the combined organization can absorb that much change on the timeline the synergy case assumes.
This is not a hypothetical failure mode. In an M&A engagement at Bain & Company, the roadmap for a post-merger integration targeted $150 million in annual synergies across three levers evaluated together (AI transformation, supply-chain globalization, and operating-model optimization), built on financial projections tested against 65+ competitors and 55+ market reports, and more than 20 expert interviews, with a post-merger organization design spanning a combined workforce of over 2,000 people. That level of diligence is what makes a synergy number survivable past the signature. A number produced without it is a negotiating position, not a plan.
What pre-deal advisory is actually for
Bringing in outside advisory before a target is locked in changes three things that are difficult or impossible to fix after the fact:
1. Pressure-testing the target list, not just the chosen target. Internal teams often evaluate the deal in front of them against the status quo, rather than against the other targets they didn't pursue. Outside advisory that isn't attached to the deal already in motion can make that comparison honestly.
2. Sizing synergies against an operating-model view, not a spreadsheet view. A synergy case built by mapping which teams merge, which decisions get one owner instead of two, and which systems actually need to be unified (versus which can coexist for eighteen months) produces a number with a real chance of surviving integration.
3. Designing the post-merger organization before the announcement, not after. Org design done reactively, once employees already know a deal closed, forces decisions under time pressure that were better made deliberately with the combined headcount and structure in view from the start.
None of this is available to a team once diligence has already started on a single target with a signed term sheet. By then, the questions worth asking before you picked this target are no longer answerable: only the questions about whether this specific deal will close on the terms in front of you.
The honest trigger
The reactive engagement (help finishing diligence, structuring the deal, or executing post-merger integration on a deal already in motion) is worth having and is what most companies call for. The higher-leverage trigger is earlier: bring in outside advisory when a company starts building an acquisition thesis, before a specific target is chosen, so the target selection, the synergy case, and the integration plan get built in that order instead of backward.
Where this fits
This is the pre-deal and integration-design work behind financial advisory: M&A, capital strategy, and CFO-level guidance built on operating-model detail, not a synergy percentage pulled from comparable deals. For capital-allocation decisions beyond a single transaction, see portfolio and capital strategy; for the organizational side of integrating two companies into one operating model, see change management.
If you're building an acquisition thesis and want the target list and synergy case pressure-tested before you commit to either, start a conversation rather than waiting for diligence to raise the questions for you.