Many mergers are pursued for reasons that are not adequately justified including:

• Overhead Reduction. Some firms consider merging for the single purpose of reducing overhead. There may be better ways to use up excess overhead, including getting rid of it. The benefits of overhead reductions are short lived and will probably not sustain a merger if the combination of the cultures is not going as smoothly as hoped.
• Bigger is Not Always Better. There are many advantages of being part of a larger operation. But bigger is not always better. Some clients are with a firm because they like the size. Some partners of smaller firms may find it difficult to accept the accountability often required by a larger firm. If “bigger is better” is the primary objective for a merger, make sure “bigger” helps the partners attain personal, financial, professional, and lifestyle goals without sacrificing clients and staff who like the current culture of the two smaller firms. Otherwise, some of these key people will not be retained. A proper transition plan can also alleviate much of the attrition risk.
• Post-Merger Activities Not Matched to Pre-Merger Goals. A firm pursuing a merger to acquire talent should be certain the target has the capability and willingness to execute the intended strategy. An example is a $2 million firm that was targeted for a merger in part because it had two excellent managers with a lot of upside potential. To take advantage of that potential, some of the managers' existing duties would need to be passed down to senior and junior level CPAs. However, after the merger, management of the combined operation failed to create the necessary lower level staff capacity. Furthermore, the managers were not coached on how to make the necessary changes in their duties. The managers became frustrated because a key objective was not achieved. Both managers left within one year of the merger.
• Speed and communication. Two of the major reasons why some mergers are not successful is 1) the speed wherein the changes are made and 2)  the lack of communication. Change is a scary thing for clients, staff and partners. Change is often necessary but wholesale changes immediately post-merger are usually not conducive to client comfort and continuity.  Slow and methodical change is more palatable for both clients and staff.  Communicating the advantages of changes, instituting them in a time frame that makes sense (some need to be accomplished day one) others can be more gradually implemented can be critical to the success of your merger

More information on pitfalls of merging:

• Mergers & Acquisition of CPA Firms: Understanding the Roadblocks to Successful Deals. (Part 1 of 2) by Joel Sinkin and Terrence Putney. AICPA, c2009
• Un-Tying the Knot: Planning For a De-Merger by Joel Sinkin and Terrence Putney. Journal of Accountancy, c2007 
• Bridging Compensation Gaps in a Merger Journal of Accountancy
• The Great Mystery: How Do Billing Rates and Profitability Affect a Firm's Worth? PCPA