Any team would have a lot of regular season success with what he inherited. Pastrnak, Bergeron, Chara, Rask, Krejci, etc. Sweeney failed in filling in the gaps. For example Sweeney really struggled for multiple years to build a good line around Krejci. That was costly in the playoffs.
His deadline acquisitions have cost a lot of first round picks and his FA signings have been generally not very good. He doesn't understand the importance of speed. Even this season the team was too slow to be a decent factor.
Aside from a couple seasons over the past decade under Sweeney the Bruins have been one and done in playoff series. Had he not walked into such a great situation already on easy mode he would have been gone a long time ago. Hopefully Boston keeps employing him because they are now in a tricky situation with the core all but gone and a group of overpaid players Sweeney signed for years to come. Makes the Atlantic a bit easier with Boston being a non factor.