I agree with everything that has been written: consultants mainly intervene on due diligences, PMI and M&A strategy.
From a different point of view, when looking at M&A service offering, consulting firms generally offer the following:
- Acquirer and Vendor Due Diligence: focus on market, competitors, target company, synergies assessment (for info, VDD are usually longer as more emphasis is given to business model and market analysis)
- PMI (pre and post merger integration): focusing on merger management, value capture, merger enablement
- Post-deal diagnosis: can focus on i) assessment of opportunities ii) first XXX days plan iii) business case and implementation roadmap
- Divestiture management: (e.g. separation strategy, operational planning & realignment,...)
Hope this helps,