The Ajax and Holland sides of the 1990s were different teams which had different expectations. I wouldn't say that Ajax really underachieved but I thought the Dutch national team could have done better.
The Red Star Belgrade team of the early 1990s didn't live up to it's full potential. In 1991, when they won the European Cup, they played great, attacking football bar the final and they had some of the best Yugoslav youngsters at that time. If it weren't for the Yugoslav civil war and for the club's money being frozen in European banks, that Red Star team could have been a powerful side.
The Yugoslav national team in the same era couldn't do much due to the same reasons. Some of those players were in Yugoslavia's 1987 Youth World Cup winning squad.