I think Hakstol is more concerned with Gostisbehere turning the puck over or getting caught up ice with no one behind him than his defensive play, per se. He just doesn't want to give up a quick breakaway-type opportunity that has a high percentage of turning into a goal when he is 5 minutes from guaranteeing a point and then turning his higher-risk players loose 3-on-3.
Last night is actually a good example, Ghost got caught chasing the puck into the offensive zone between the faceoff dots, the goalie pokes it to his defenseman, and the Stars' defenseman takes advantage of the Flyers' defenseman (Ghost) getting caught way up ice by threading a long pass all the way up to the middle and giving the Stars a game-winning 2-on-0.
Luckily, the Flyers' still get a point because it was OT, and I'm sure Ghost was playing it even riskier than usual because of 3-on-3, but the fact remains he *does* make high risk plays as a defenseman, that's his style, and he does sometimes get caught, leading to breakaways. It's happened plenty of times throughout his career.