Two CEOs sit down, culture had better be the #1 thing they discuss before one writes a check to the other for the better part of half a billion dollars.
Good M&A adds tons of value by combining two companies who are better together than separate. Bad M&A (Google's acquisition of Nest comes to mind) means fired CEO, delayed/ruined product launches, and incineration of nine-figure amounts of shareholder value. It all comes down to whether the two companies want the same thing.
I won't argue that Atlassian software has no rough edges UX-wise, but that isn't the point. The point is that, when one company buys another, they'd better have roughly the same idea of where they want to go, otherwise you may as well fire up an industrial blast furnace fed by shareholder capital.