I wouldn't categorize it a failure at all, but I would certainly acknowledge some of these negatives:
1) He did play for most of the year but he also got injured at the time we needed him the most. He also was clearly on some restrictive plan during the regular season that prohibited increased minutes in case we needed him to fill in and play a bigger role. It's not totally his fault, but if you were concerned about his health going in, you might have a case.
2) All year people touted his defense and how he fit our switching philosophy. While he wasn't unplayable in the playoffs, he definitely was challenged a lot by the best players and he didn't always hold up as well as expected.
3) People did want him to be a third offensive threat but some also expected him to improve our playmaking. However, he either wasn't good at it or he simply wasn't asked to play that role by the coaching staff. It seems like he was asked to do two things, mostly: shoot spot-up 3s, the occasional step-back, or drive to the basket. It didn't seem like he was often involved in the offense beyond that. I don't know how much of that was his own limitations, his teammates (Tatum) wanting to control the offense, or the aforementioned offensive plan, but it just didn't happen as much as some thought it would.
4) If you were one of the people (like me) that wanted them to prioritize finding Horford's replacement/Rob insurance last offseason, it could be quite vexing to see Horford flailing around out there. Then you had the three guard lineups where they simply looked way too freaking small. That draft pick and the salary slot could have been used in a different way, but they clearly prioritized an offensive guard due to what happened vs. the Warriors. With White improving his overall offensive play it became less of a critical need this year
than we thought it was.