Here's a more objective approach: rank each player by win shares in their draft class. It's not perfect but it's better than opinions, which is all we've gotten so far.
I'll throw out top 10 picks to be consistent with the post above. You'll see the player's name, their draft position, and their WS rank in the class, followed by "OVER" or "UNDER" perform. I'll call it "EVEN if the rank is +/- 1 slot.
Also looking at 10 years of history.
2017: Rob Williams, 27 (21), OVER
2017: Semi Ojeleye, 37 (25), OVER
2017: Kadeem Allen, 53 (36), OVER
2017: Jabari Bird, 56 (46), OVER
2016: Ante Zizic, 23 (24), EVEN
2016: Abdel Nader, 58 (30), OVER
2016: Guerschon Yabusele, 31 (16), UNDER
2016: Demetrius Jackson, 45 (38), OVER
2016: Ben Bentil, 51 (50), EVEN
2015: Terry Rozier, 16 (12), OVER
2015: Jordan Mickey, 33 (36), UNDER
2015: RJ Hunter, 28 (38), UNDER
2015: Marcus Thornton, EVEN (never played but drafted in a slot where never playing is the norm for that year)
2014: James Young, 17 (37), UNDER
2013: Kelly Olynyk, 13 (8 ), OVER
2012: Jared Sullinger 21 (19), OVER
2012: Fab Melo, 22 (48), UNDER
2012: Kris Joseph, 50 (51), EVEN
2011: ETwuan Moore, 55 (22), OVER
2011: JaJuan Johnson, 27 (45), UNDER
2010: Avery Bradley 19 (15), OVER
2010: Luke Harangody, 52 (31), OVER
2009: Lester Hudson 58 (45), OVER
2008: Semih Erden 60 (37), OVER
2008: JR Giddens, 30 (45), UNDER
So what does this leave us with?
UNDER: 7
OVER: 14
EVEN: 4
These are great numbers: Ainge drafts guys that outperform their draft slot twice as often as he drafts guys that underperforms.
You could do these using different metrics, but I doubt you'll see too much difference in the overall conclusion. The claim that Ainge is a terrible drafter is just 100% wrong. Because this is a zero-sum method, for every pick he makes that outperforms, there must be a GM out there who's underperforming.
For what it's worth I had done another one of these several years back, going all the way back to Ainge's beginning with the organization, and you see the same thing. There are just a handful of guys who are arguably better than him. (Rondo is the best #21 pick EVER, unless you want to put Michael Finley above him, and Al Jefferson is one of the 5 best picks at his slot as well.)
I can't see into other people's heads, but I suspect that a lot of the knee-jerk thinking about this comes from one thing: not understanding exactly how few players at any draft slot end up being legit NBA players, much less stars. That's probably a salience effect: we remember Giannis as a 15 pick, but forget that the typical player in that range is someone like Adriean Payne, or Kelly Oubre, or Royce White, or Lucas Noguiera, or Shabazz Mohammad...
I think the only way to really get a sense for that is by spending time looking at the drafts, going back years.