I was involved in PMIs where, as you put it, the case was strong – from the POV of strategic, financial and cultural fit. Post-closing however, something went wrong. Looking back, part of the reasons was that at pre-acquisition, there were material information not available to the deal team which emerged only post acquisition. In some cases, the deal team was able to address by adjusting the integration plan. They also adjusted the target synergies to a realistic level. But in other cases, failure was also due to the fact that the motivation / push from the leadership to realise the synergies was simply inadequate.
In my experience, one of the major reasons M&As fail is because of disproportionate emphasis on pre-deal preparation; whereas the most critical aspect, i.e. integration is sidelined or becomes rather an afterthought.