The combination of cheap financing and expensive stocks makes this an attractive time for dealmakers, but investors have to be more cautious about mergers and acquisitions (M&A) that too often end up as value destroying disappointments. “The open secret about M&A is that most deals fail to generate the synergies companies expect when they announce a merger,” write Laura Miles, Adam Borchert, Alexandra Egan Ramanathan for Bain & Company. “Most merging companies entering a deal don’t have a clear understanding of the level of synergies they can expect through increased scale. They typically make broad estimates based on prior deal announcements,…