How big a deal and how common is it in reality? "We have one fewer spots because we admitted the son of a donor" and "we won't accept you based on race" are different magnitude.
https://admissionsight.com/harvard-legacy-acceptance-rate/ says it's 25-35% of students who're legacies at Harvard.
https://www.nber.org/system/files/working_papers/w26316/w263... is analyzing the admissions data that got released from an earlier lawsuit against Harvard about Asian American admissions bias, and says that although being a legacy isn't as good as being a recruited athlete, it still gets you admitted at about 5x the rate of non-legacies.
They're vastly different in terms of magnitude.