Sullinger (until he ate his way off the team) and RWIII are both late round picks that have/are out performing their draft position, and Semi in the 2nd has out performed his draft position. Then there are Glenn Davis, Dwight Powell and Etwan Moore who were DA players. The KO pick gets killed because of Giannis but KO was a good pick where he was drafted. Pritchard is looking like a plus pick for the position also.
Not every pick is a homerun but that's solid drafting.
Ainge has definitely found guys who exceed the value you'd expect from their draft slot.
That's not the same thing as finding a real gem, though.
Ainge has not found a star in the later parts of the draft, e.g. Giannis, Gobert, Draymond, Jokic, Siakam, Adebayo, etc.
Now, to be clear, there isn't a guy like that in every draft, and even when there is, usually at least half the teams in the league pass on the guy before he's drafted. By definition that's how a guy ends up being a steal. So it's not some black mark that Ainge hasn't found one.
At the same time, you could argue that finding a major hidden gem like that is the one remaining avenue the Celts have to go from a good-not-great team to one of the premier teams in the league, now that their young stars are on their second contracts and the cap sheet looks to be pretty full-up for the foreseeable future. The fact that Ainge hasn't managed to do it in a long time is at least somewhat concerning.