I (a Pens fan) saw Sullivan mentioned in the OP...
He did an outstanding job during his first two years (both leading to Cups), but his approach has come into question since. He continues to instill a system based on speed, even though the Pens do not have the legs for it. Do the Bruins have the legs to apply pressure in all three zones? His line combinations are hit or hiss.
Speaking of which... He aslo stumbled upon the HBK line before the first Cup. He put the line together last minute when Malkin was out of the lineup. Do the Bruins have the depth to put a premier goal scorer (Kessel) on the third line?
And like any other Pens coach, he has two generational players (and a game-breaking defenseman) to defer to.
I realize some Bruins fans may be having flashbacks to him being a first time coach overseeing a core in it's prime (Thornton, Samsonov, etc.), but he's in the same group as Lavy. He's only good with a veteran group that is ready to compete.