Talent retention is extremely tricky and warrants a significant amount of effort, instead of downplaying its importance. I have seen both sides of the spectrum, where the quantum of retention grants awarded to selected leadership is well over market prevalence, and I have witnessed situations where retention bonuses are uniformly spread across the workforce without much thought on succession planning, only to eventually exit individuals from an organization. HR should be involved every step of the way and should voice opinions on talent. While outsourcing may be helpful, it is always best to get inside opinions from HR professionals of the buyer who truly understand what is it like to live the culture.
While all functions wish to achieve the same objective of closing the deal, it also puts pressure on getting across the finish line which may lead to short-term decisions driving long-term outcomes.