Cultural differences. Pick any survey of reasons that get cited for "failed" M&A, and you will likely encounter cultural differences amongst the most cited reasons.
Cultural differences is a very peculiar term - it is not too difficult to grasp and at the same time, near impossible to describe objectively. It therefore usually remains in the realm of - the buyer and the target approach the same thing differently, people in these organizations prioritize different outcomes and the means to achieving them, so much so that there is little consensus post M&A on goals and approaches and anything that gets pushed down on either organization ends up backfiring and things come to a naught.
In this post, let's try to bring some objectivity into this situation.
Firstly - cultural friction is not an M&A-only phenomenon
Cultural differences, as it usually gets defined, happen everywhere and all the time. Within the same organization, different teams and functions complain of cultural differences all the time. Sales people detest product people who cannot get along with engineering. Management does not seem to have their finger on the pulse of the larger organization. Millennials have their work ethic, gen Z have theirs. And so on. Look close enough and fault lines like these are all pervasive. It is not as if you have these challenges only in M&A. Now, if an organization were to go down, you will seldom hear cultural issues being blamed. But with M&A, somehow there is an eagerness of play up the cultural differences angle as a top reason for when things fall short.
Secondly - what gets called "culture" is often just a turf war
And no, if you are to think that the intensity of cultural differences is much higher in a post M&A situation than during routine business, then here is the second point. Cultural differences, being the amorphous & opaque term that it is, often gets in the way of a deeper and better understanding of what happens in post M&A situations. Often, there are turf wars that erupt between the buyer and target orgs and that is both understandable and natural. The buyer wants to impose its writ visibly and the target wants to preserve the way it operated pre M&A. This is about power. This is reflexive behaviour - you pull this way, I go the other way. This is as human as it gets. And this is as old as humanity. And the post M&A period is one where the jockeying for turf is intense. And in that intense period, lots of things get played up, including differences between teams and organizations. Good managements know that turf wars will happen post M&A. They also realize that it cannot really be addressed fully pre M&A, regardless of how well you carve out roles or how well the leadership of both organizations might get along during the M&A process. But, to confuse all of these for some inherent cultural differences is losing the plot.
Thirdly - differences are not inherently bad
Even if cultural differences do exist between the buyer and the target organizations (and not being played up like in the second point), it should not come with the assumption that such differences are just plain bad and undesirable and will only lead to bad outcomes. This is just overdoing things and unwarranted wallowing in pathos. Different businesses in a group operating differently does not have to be seen as a bad thing. We often underestimate the importance of an organization's practices and its way of doing things, in its success. For the management, there is value in just letting things be. There are better and more definite things to align on or standardize, for success after M&A.