Some hints: Strategy usually is not the problem; sometimes financial projections don’t include important considerations; and almost always it’s execution that falls short.
At the start of a current research project I’m working on [see below if you want to participate] I did the proverbial Google search on “reasons for M&A failure” to see if anything has changed recently. My search returned 1.84 million results in under half a second. When I narrowed the search to include only studies, articles and blog posts since 2015, Google returned a pared down 553 thousand results. In reading the major research and scanning the headlines of other entries, there were not new topics on this list.