Importance of IT M&A activities on Deals (carve-outs, integrations, spin-offs)

All big companies such as Ford, Facebook, GE, American Airlines, are providing their products/services and running their operations by the use of IT systems. Due to the fact that almost all business processes (Finance, HR, supply chain, production, etc.) are supported by information technology (systems, applications, etc.). The fact is that IT is quite complex to understand (firms might have lots of apps, servers, data centers, etc.) and is often one of factors that many acquisitions are not successful (after integration of 2 companies, their systems might not talk to each-other, they might have few instances of ERP systems, data might not be correctly integrated, etc.). What are important IT M&A considerations on M&A Deals (pre-Day 1 and post-Day1). What can we do to avoid IT problems / risks during M&A activities?