Sometimes because Fergie was so successful at Utd, his achievements at Aberdeen have actually been underrated or even almost ignored on occasions. Aberdeen won as many major trophies during his time in charge there (including 3 out of their 4 league titles, 4 out of their 7 Scottish Cups and 100% of their European silverware), as they did during the remainder of their history before and after his reign.
For example when Chelsea won the title in 2015, the 8th league title of Mourinho's managerial career, there was some talk about whether he could potentially equal / overtake Fergie's 13 league titles (even then I didn't think that he was going to win another 5 league titles). The problem was that of course Fergie won 16 league titles during his managerial career not 13, and the 3 that he won at Aberdeen were being ignored.