What is the key success factor for M&A

In fact, my belief is that there is always a value for the company being acquired, otherwise the acquiring entity will not spend money to acquire that company. But what is the reason why when the acquired company is merged under the acquiring company then everything goes in the opposite direction of initial plan?

I think it could come from few aspects, namely human judgement error or biasness, hidden agenda, inaccurate data and asymmetric information.
It is not uncommon that people always have errors in judgement or biasness in their thinking, especially for some managements, since they are holding up high position and they have to materialize what they have devoted or resources deployed, even though they realize their mistake or biasness during the half way of M&A process, sometimes, they still go ahead with the entire process otherwise their reputation will be doubted or questioned. secondly, they do not set the right hypothesis at the beginning, or the hypothesis set are not fully relevant to the core of the issues. thirdly, they do not get the high quality of data to testify their hypothesis and eventually the decision was misled. Last but not least, the entire M&A process was surrounded by a bunch of people, and sometimes different people has different agenda and that situation will deviate the original aim and purpose of the M&A and eventually the result was at the opposite direction of initial goal.