I don't think Sweeney has been a very good GM over the years.
This may be an unpopular opinion, but I think the Bruins were as good as they were from 2011-2024 because they usually had very good coaching and not necessarily because their roster was full of great players.
Their team performance was usually above the sum of their individual parts, and that usually happens because of great coaching.
Cassidy and Montgomery were scape-goated because they stopped being able to work miracles with an average roster.