How many can you really give him credit for?
He was hired as director of player development in June 2006, promoted to director of hockey ops in 2007 and assistant GM in 2009.
So he really had nothing to do with bringing in Bergeron, Krejci, Chara, Rask, or Marchand.
So that leaves Pastrnak and Gryz who were both brought in while he was Assistant GM. Toss in Krug too.
Fact is, the core group of this team since 2008ish he inherited.
He was part of some hits at the draft table. 2014-2017 he was part of finding some good players even if their were some glaring misses. The 2018-2020 drafts will hurt both on the players chosen, the lengthy timeline for development, and the picks traded away.
His big name UFA acquisitions have been poor. Belesky and Backes didn't work out. John Moore became a spare part quickly and his most useful quality would be shipping him elsewhere for cap space. We'll see if handing Craig Smith 3-years was a prudent move.
On the flip side, he's done decent on under-the-radar low cost UFA signings (Riley Nash, Nordstrom, Schaller, Halak, Khudobin, etc.)
His first year of trades was pretty awful both that summer and the 2016 deadline which he chose to hang onto Eriksson and trade away a bevy of picks for mediocre help when it was clear that team wasn't a contender (proven by the fact they didn't even make the playoffs).
He ended up paying a fairly hefty price for Rick Nash, but he couldn't of foreseen the concussion. He made two very nice deals in 2019 for Coyle (his best trade by far) and Mojo. The jury is still out on his Kase and Ritchie deals. He practically never makes any trades during the season, choosing only to deal around July 1st and the trade deadline.
Without the bevy of talent acquired from June 2003 to July 1st 2006, he'd be in big trouble, no doubt about it. But on the other hand, he's done very well at extending the Bruins own guys to very nice deals.
That's a pretty big mixed bag when you look at his drafting, signings, and trades. If you were grading Sweeney on a scale of 1-5 (1 being poor and 5 being great), he's basically a 3.