Terrific question @najranmt! I am guessing that you are asking what are the most common challenges, so please correct me if I am incorrect.
The root of the most common challenges is Human Capital Due Diligence. This includes talent assessment, identification of key personnel to retain, evaluation of human capital programs (total rewards [benefits & compensation], performance management, retention, engagement, and L&D), and development of communication and retention strategies through the M&A engagement. While direct costs are always a key focus of the financiers throughout the engagement, not enough attention is paid to the long-term impact of the human capital components, some of which may cost a bit more upfront but pay significant dividends in the 1-3 year period post transaction close.
@QAHTFA00 is correct, that one of the most common challenges is “losing key employees from the target company”. The loss of key talent is often accompanied by the loss of intellectual property and institutional knowledge that was not appropriately transferred. With the appropriate talent assessment, identification of key employees, evaluation of human capital programs with a critical eye on those that best advance the transaction thesis and merged entities strategic plan, and carefully crafted communication and retention plans all performed and developed during the Human Capital Due Diligence phase, you will significantly increase the success of the transaction in the critical 1-3 year window post close.