I think Ainge has a been a good drafter overall, but had some pretty serious misses in the "teens" for whatever reason (as I've pointed out a few times). So far, Nesmith and Langford are not exactly bucking this trend, sadly.
But to Ainge draft defenders, I'd just repost that no player Ainge has ever drafted outside then Top 3 has ever made an All-Star game, other than Rondo (back in the ice age). And he's had a lot of cracks at it.
In a league where championships are won by stars, finding gems is critical -- especially for stronger franchises that don't live at the top of the lottery over and over again. Read: Utah, San Antonio, Toronto, Denver, Milwaukee, etc. Ainge just hasn't been able to do it, which is a little surprising considering how savvy he is elsewhere.
So, the trade that landed them the picks to select Brown, then Tatum? Gem. Selecting Brown? Gem. Trading down and taking Tatum? Gem. But Ainge just hasn't been able to find those kinds of hidden ones later in the draft.