From personal experience and from what I have heard from fellow colleagues, I believe there are two main reasons for unsuccessful M&A.
The first one is related to the very first stage of the M&A process, prior to merging and acquiring and that is the development of M&A strategy and choosing candidates for M&A. At this stage, leaders and strategists determine the high-level goals for the firm and what they seek to gain from this transaction, such as increasing share in their current market or expanding to new ones. It can happen that the very strategy they aimed for from the begining wasn’t well defined or wasn’t profitable, and therefore the whole process can end up being unsuccessful. If valuation analysis isn’t done properly it could simply lead to choosing the wrong candidate for the M&A.
The second reason, is due diligence, as highlighted here https://dealroom.net/faq/what-is-m-a-process-everything-you-need-to-know Not conducting due diligence properly can result in unsuccessful M&A because the Once an offer with the final candidate is on the table, the acquirer must conduct due diligence. If they don’t carefully examine, evaluate, and analyze all aspects of the target company’s operations and financial position prior to establishing a definitive agreement and there happen to be important threats for the future of the M&A deal with that candidate, the whole deal can result in failure after a process is already completed.