Well, I rated Anderson when he arrived at Utd, and I don't think I was wrong in a footballing sense - his cheery energy and willingness to drive forward were genuinely enjoyable to watch. He had pace and great technical ability. What you can't account for are character failings - in Anderson's case, but the same would apply to a lot of other overrated players, weak mentality and lack of personal discipline, aggravated by cruciate injuries (possibly because of the same lack of discipline) and pure bad luck (he was in a serious car crash). The same applies to, for example, Januzaj, who showed every sign technically of being a top player but coped badly with his elevation to the first team and took a long time to mature.
Both failed to fulfill their potential - there's still time left for Januzaj - but I don't think I overrated them at the time. The truly overrated players, for me, are the likes of Pogba, whose statistics, we are told, prove their high rating but somehow only manage to exert a baneful and negative influence on the team.