I would say Edberg's balance sheet is pretty even. As others have noted, there were some Majors he could have/should have won.
OTOH, there were some Majors he won that he easily could have lost:
1985 Australian Open: In the SF, Edberg faced a more experienced Lendl, who was coming off a win at the U.S. Open, his second Major title. Meanwhile, this was Edberg's first Major SF. Edberg was able to eke out a win 9-7 in the fifth set before taking the title against Wilander in the final.
1987 Australian Open: In the final, Edberg coughed up a two set lead against Cash but was able to break early in the fifth set. With Edberg seeming to visibly tire, Cash collected break points in two different games but squandered them with mostly bad play, rather than Edberg really stepping up.
1990 Wimbledon: He was down 2-1 in sets to Mansdorf, who held with more ease during most of the fifth set before Edberg pulled out a 9-7 win. Of course, we all know about the final, where Becker came from 2-0 down in sets before being up a break in the fifth set, with Edberg eventually coming back.
1992 U.S. Open: A point here or there and he loses the five set matches he won against Lendl and Chang. Not sure how close he was to losing the five set win he had over Krajicek. In the final, Sampras served for a 2-1 set lead before Edberg broke and won in four sets.
So, yeah, there were some squandered opportunities by Stefan, but also some gutsy matches to take 4/6 Majors he won.