Board of Directors vs CEO ExCom role in a proposed M&A

What is and should be the role of the Board of Directors compared to the role of teh CEO and ExCom in view of a decision to a proposed M&A?
I am interested to hear your views as I am puzzled by the large number of failed M&A’s that the related statistics and experience show and would like to understand more from the participants in this course about their past real experiences.
As it is true that most of the times BoD’s rarely object on a proposed/decided activity by the CEO and the ExCom and also rarely evaluate ex post the results vs the assumptions and commitments, either in terms of value or synergies.

Thank you