The suggestion is that they do fail.
The reason is that firms do not integrate properly so that the clients of the merged business do not get any benefit from the merger, and if anything suffer from the dislocation that the merger causes.
Three firms that seem to be suffering from a failure to integrate effectively are DWF, Shakespeares and Slater & Gordon.
The majority of law firm mergers fail or run into difficulty because of a lack of a coherent plan to capitalize on the combined client relationships. Management efforts pre- and post-merger are instead typically too narrowly focused on integrating operational aspects such as technology platforms and billing systems. “Making sure that everyone has the same email address isn’t what will determine whether a merger is successful," said Edsberg, a lawyer turned consultant who spent six years as an attorney at Freshfields Bruckhaus Deringer. "The litmus test of any merger should be whether the combined firm can bring value to clients in a way that the old firms couldn’t on their own.”